A few items which I would find helpful in the UV editor:
It would be nice if it had a bit more live feedback. For instance, when moving / scaling / rotating a render mesh in the UV editor, if I could see the updates happened in real time on the model. Currently it doesn’t show until I have finished the move / scale / rotate. This would be handy for understanding how the island / model relate.
Currently, when I select a render mesh, that mesh highlights on the model (which is great). It would be nice if the reverse worked as well. Further, if I subselect a surface / mesh face on the model, that render mesh would be highlighted in the UV editor. It would also be nice if I could subselect say an edge in either the UV editor or the model, and I could see that subselected edge in the other space. This would be very handy for determining the orientation of the render mesh in the UV editor. It would also be nice if both the UV editor and the object you are editing respected the Shade-highlight selected meshes / surfaces / polysurfaces setting in the display mode.
It would be nice if Unwrap and the UV editor were a bit more integrated. For instance, if I’ve unwrapped something, and when I go to the UV editor and I see I should have picked another edge for a seam. It would be nice if in the UV editor, I could select a mesh edge I would like to be a seam, and it could re-unwrap right in the editor instead of having to go back and forth. This would also be very handy for meshes, which don’t seem to remember previousSeamSelection in the Unwrap command.
It would be nice if we could see how much something was stretching in the UV editor, perhaps a display option similar to the curvature analysis, but instead of showing a gradient of colors for how much a surface is curving, it is showing a color gradient for the pre-unwrap surface area vs the post unwrap surface area.
It would also be nice if there was some tool for visualizing the comparative number of pixels each island consumes. By this I mean if I want to make sure that each island that I have defined via unwrap consumes a UV area proportionate to the surface area of the island in model space. Often I find I have and island of some random end of an object that is like 5 times the size of an island for a face of the object which is much larger on the model. It would be nice if there was some easy tool that would tell me “hey, this island is way out of scale compared to how much area in the model it is taking up.”
It would be nice if there was some sort of overlapping UV warning, and even a island border incursion warning, like if I wanted to ensure a 5 pixel border around my islands. Same for the gutter.