Yes, another possible enhancement would be expression debugging. 

It turns out the Location class of JDPA only has the line numbers and code 
index. The code index does nto necessarily map to statements/expressions. This 
is even more so when the code is optimized. Therefore it may not be possible 
to support Step over statement/expression unless JPDA introduces that support 
in the location class.

Anyhow, it should be possible to support "Step over code index". The user may 
not see any difference though, as the highlight in the editor will stay on the 
same line if the next code index is on the same line. If the editor has an 
ability to show bytecode view also (which I have seen in some Java IDE's as 
well as older C/C++ debuggers/IDEs) this could be useful. The code 
optimization as well as JIT and Hotspot always makes some of this harder.

The new kind of step is under development, we need an API review, before it can
be put into trunk. See issue #93842.
The implementation of so-called expression-step is on AST branch in debuggercore
and debuggerjpda modules.

The expression-stepping implementation is in trunk now. The code from AST branch
was merged into trunk:

Checking in debuggercore/api/apichanges.xml;
/shared/data/ccvs/repository/debuggercore/api/apichanges.xml,v  <--  apichanges.xml
new revision: 1.7; previous revision: 1.6
done
Checking in debuggercore/api/manifest.mf;
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new revision: 1.14; previous revision: 1.13
done
Checking in debuggercore/api/src/org/netbeans/api/debugger/ActionsManager.java;
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 <--  ActionsManager.java
new revision: 1.22; previous revision: 1.21
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Checking in
debuggercore/src/org/netbeans/modules/debugger/resources/Bundle.properties;
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new revision: 1.15; previous revision: 1.14
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Checking in
debuggercore/src/org/netbeans/modules/debugger/resources/CurrentExpression.xml;
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new revision: 1.2; previous revision: 1.1
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Checking in
debuggercore/src/org/netbeans/modules/debugger/resources/CurrentExpressionLine.xml;
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new revision: 1.2; previous revision: 1.1
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Checking in debuggercore/src/org/netbeans/modules/debugger/resources/mf-layer.xml;
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Checking in
debuggercore/src/org/netbeans/modules/debugger/resources/actions/StepOverOperation.gif;
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new revision: 1.2; previous revision: 1.1
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Checking in
debuggercore/src/org/netbeans/modules/debugger/resources/actions/StepOverOperation24.gif;
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new revision: 1.2; previous revision: 1.1
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Checking in
debuggercore/src/org/netbeans/modules/debugger/ui/actions/Bundle.properties;
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Checking in
debuggercore/src/org/netbeans/modules/debugger/ui/actions/DebuggerAction.java;
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 <--  DebuggerAction.java
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Checking in debuggerjpda/arch.xml;
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new revision: 1.9; previous revision: 1.8
done
Checking in debuggerjpda/ant/nbproject/project.properties;
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<--  project.properties
new revision: 1.10; previous revision: 1.9
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Checking in debuggerjpda/ant/nbproject/project.xml;
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project.xml
new revision: 1.22; previous revision: 1.21
done
Checking in debuggerjpda/api/apichanges.xml;
/shared/data/ccvs/repository/debuggerjpda/api/apichanges.xml,v  <--  apichanges.xml
new revision: 1.18; previous revision: 1.17
done
Checking in debuggerjpda/api/manifest.mf;
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new revision: 1.20; previous revision: 1.19
done
Checking in debuggerjpda/api/src/org/netbeans/api/debugger/jpda/CallStackFrame.java;
/shared/data/ccvs/repository/debuggerjpda/api/src/org/netbeans/api/debugger/jpda/CallStackFrame.java,v
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new revision: 1.10; previous revision: 1.9
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Checking in debuggerjpda/api/src/org/netbeans/api/debugger/jpda/JPDAStep.java;
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 <--  JPDAStep.java
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Checking in debuggerjpda/api/src/org/netbeans/api/debugger/jpda/JPDAThread.java;
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 <--  JPDAThread.java
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Checking in debuggerjpda/api/src/org/netbeans/spi/debugger/jpda/EditorContext.java;
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 <--  EditorContext.java
new revision: 1.10; previous revision: 1.9
done
Checking in
debuggerjpda/projects/src/org/netbeans/modules/debugger/jpda/projects/AST2Bytecode.java;
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 <--  AST2Bytecode.java
new revision: 1.2; previous revision: 1.1
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Checking in
debuggerjpda/projects/src/org/netbeans/modules/debugger/jpda/projects/DebuggerAnnotation.java;
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 <--  DebuggerAnnotation.java
new revision: 1.2; previous revision: 1.1
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Checking in
debuggerjpda/projects/src/org/netbeans/modules/debugger/jpda/projects/EditorContextImpl.java;
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 <--  EditorContextImpl.java
new revision: 1.5; previous revision: 1.4
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Checking in
debuggerjpda/projects/src/org/netbeans/modules/debugger/jpda/projects/ExpressionScanner.java;
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 <--  ExpressionScanner.java
new revision: 1.2; previous revision: 1.1
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Checking in
debuggerjpda/projects/src/org/netbeans/modules/debugger/jpda/projects/ToolTipAnnotation.java;
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 <--  ToolTipAnnotation.java
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done
Checking in
debuggerjpda/src/META-INF/debugger/netbeans-JPDASession/org.netbeans.spi.debugger.ActionsProvider;
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 <--  org.netbeans.spi.debugger.ActionsProvider
new revision: 1.5; previous revision: 1.4
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/EditorContextBridge.java;
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 <--  EditorContextBridge.java
new revision: 1.9; previous revision: 1.8
done
Checking in debuggerjpda/src/org/netbeans/modules/debugger/jpda/ExpressionPool.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/ExpressionPool.java,v
 <--  ExpressionPool.java
new revision: 1.2; previous revision: 1.1
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/JPDADebuggerImpl.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/JPDADebuggerImpl.java,v
 <--  JPDADebuggerImpl.java
new revision: 1.112; previous revision: 1.111
done
Checking in debuggerjpda/src/org/netbeans/modules/debugger/jpda/JPDAStepImpl.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/JPDAStepImpl.java,v
 <--  JPDAStepImpl.java
new revision: 1.5; previous revision: 1.4
done
Checking in debuggerjpda/src/org/netbeans/modules/debugger/jpda/SourcePath.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/SourcePath.java,v
 <--  SourcePath.java
new revision: 1.8; previous revision: 1.7
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/actions/StepActionProvider.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/actions/StepActionProvider.java,v
 <--  StepActionProvider.java
new revision: 1.33; previous revision: 1.32
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/actions/StepIntoActionProvider.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/actions/StepIntoActionProvider.java,v
 <--  StepIntoActionProvider.java
new revision: 1.31; previous revision: 1.30
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/actions/StepOperationActionProvider.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/actions/StepOperationActionProvider.java,v
 <--  StepOperationActionProvider.java
new revision: 1.2; previous revision: 1.1
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/breakpoints/ClassBasedBreakpoint.java;
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 <--  ClassBasedBreakpoint.java
new revision: 1.18; previous revision: 1.17
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/breakpoints/MethodBreakpointImpl.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/breakpoints/MethodBreakpointImpl.java,v
 <--  MethodBreakpointImpl.java
new revision: 1.16; previous revision: 1.15
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/CallStackFrameImpl.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/CallStackFrameImpl.java,v
 <--  CallStackFrameImpl.java
new revision: 1.22; previous revision: 1.21
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/JPDAThreadImpl.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/JPDAThreadImpl.java,v
 <--  JPDAThreadImpl.java
new revision: 1.22; previous revision: 1.21
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/LocalsTreeModel.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/LocalsTreeModel.java,v
 <--  LocalsTreeModel.java
new revision: 1.40; previous revision: 1.39
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Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/ReturnVariableImpl.java;
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new revision: 1.3; previous revision: 1.2
done
Checking in
debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/ThisVariable.java;
/shared/data/ccvs/repository/debuggerjpda/src/org/netbeans/modules/debugger/jpda/models/ThisVariable.java,v
 <--  ThisVariable.java
new revision: 1.8; previous revision: 1.7
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Checking in
debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/ExpressionStepTest.java;
/shared/data/ccvs/repository/debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/ExpressionStepTest.java,v
 <--  ExpressionStepTest.java
new revision: 1.2; previous revision: 1.1
done
Checking in
debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/test/TestContextProvider.java;
/shared/data/ccvs/repository/debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/test/TestContextProvider.java,v
 <--  TestContextProvider.java
new revision: 1.13; previous revision: 1.12
done
Checking in
debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/test/TestEngineContextProvider.java;
/shared/data/ccvs/repository/debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/test/TestEngineContextProvider.java,v
 <--  TestEngineContextProvider.java
new revision: 1.12; previous revision: 1.11
done
Checking in
debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/testapps/ExpressionStepApp.java;
/shared/data/ccvs/repository/debuggerjpda/test/unit/src/org/netbeans/api/debugger/jpda/testapps/ExpressionStepApp.java,v
 <--  ExpressionStepApp.java
new revision: 1.2; previous revision: 1.1
done
Checking in debuggerjpda/ui/nbproject/project.xml;
/shared/data/ccvs/repository/debuggerjpda/ui/nbproject/project.xml,v  <-- 
project.xml
new revision: 1.17; previous revision: 1.16
done
Checking in
debuggerjpda/ui/src/META-INF/debugger/netbeans-JPDASession/org.netbeans.spi.debugger.ActionsProvider;
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 <--  org.netbeans.spi.debugger.ActionsProvider
new revision: 1.5; previous revision: 1.4
done
Checking in
debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/resources/mf-layer.xml;
/shared/data/ccvs/repository/debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/resources/mf-layer.xml,v
 <--  mf-layer.xml
new revision: 1.35; previous revision: 1.34
done
Checking in
debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/EditorContextBridge.java;
/shared/data/ccvs/repository/debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/EditorContextBridge.java,v
 <--  EditorContextBridge.java
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Checking in
debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/SourcePath.java;
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 <--  SourcePath.java
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done
Checking in
debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/models/Bundle.properties;
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 <--  Bundle.properties
new revision: 1.36; previous revision: 1.35
done
Checking in
debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/models/VariablesNodeModel.java;
/shared/data/ccvs/repository/debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/models/VariablesNodeModel.java,v
 <--  VariablesNodeModel.java
new revision: 1.14; previous revision: 1.13
done
Checking in
debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/models/VariablesTableModel.java;
/shared/data/ccvs/repository/debuggerjpda/ui/src/org/netbeans/modules/debugger/jpda/ui/models/VariablesTableModel.java,v
 <--  VariablesTableModel.java
new revision: 1.18; previous revision: 1.17
done

Verified ... and Closing all issues resolved into NetBeans 6.7 and earlier.