My idea:
UI and sound edits! Community-created themes either added officially to the game or usable as mods. These would allow for their own settings for all the elements, editable in the HUD editor or in a different menu entirely. This could be used for different Sounds or interfaces that are intended for more serious players, such as minimalizing certain assets or changing sounds for things they want to hear more easily or not hear at all, interfaces and sounds for the nostalgia-hungry, or just new interfaces and sounds for people looking for a new twist to the game!