This picture basically says it all. Add a button to swap between osu/taiko/ctb/mania and edit mode. I always find myself testing a section of a map in edit mode while learning how to play the map or just seeing how notes and such are spaced before going back to play it. Also, there are often people I talk to that like to spectate after an update so being able to swap conveniently would be nice.
TL;DR? I'm lazy and I'd like to be able to swap modes without having to go to the title screen every time
also before I forget.. you'd need to add the mode button on the edit screen for this to actually be useful
also.. I do know about right clicking a map to edit it. Just can't do it the other way around. another suggestion could be to add a 'go to play mode' button on the right click.