I have a few things that I would like to see that I think would be useful. If I suggest something that's already implemented or would be too much trouble with too little benefit, I apologize ahead of time.
-Pan being integrated as a keyboard shortcut.
-Icon subsets would be useful. Instead of going into the build menu to change from a dodecahedron to an icosahedron, perhaps something like right-clicking the icon would bring up all primitive variants that are related to it (in this particular case the platonic solids). This could also be integrated into the edit mode tools to provide variants on some operations if necessary. In particular though, this might be useful for organizing the menu for those of us with an endless amount of primitive plugins.
-Free-form select.
-For many of the input box tools like smooth object, rotate custom, primitive parameters, etc., having a slider in addition to it that automatically updates the model to reflect the outcome before completing the operation would be useful for visualization and save a bit of guess work.
-Changing the color of GUI components could be useful, for personal preference or otherwise. In my case, a lot of the reference images I use tend to have white backgrounds, so being able to change my selected point/edge color to something that blends in less would save a bit of hassle in the long run. Along with that, things like scaling windows could be useful as well when dealing with multiple views.
-Steve: Implemented in 1086: Transparency as a part of the reference image parameters. That, or being able to have objects always draw in front of a reference image.
-Rotate faces obeying the x,y,z restrictions like the point rotate would be useful when working with extrusions.
I don't feel that any of these should be too high on the priority list, but they are some things that I feel would benefit the program without over-complicating it.