Okay, not all of them might be quick to implement, but they're certainly quick to describe:

* For those folks with 4 weapon slots, make them all keybindable individually.
* Jingles should be on music volume level, not sound effects
* Single heart health boni should be adjusted for later depths - same for "make your choice" group switches, which really are a no-brainer at T3+
* Give us an overview map (keybindable) - doesn't need to show unexplored stuff, but does need to show as of yet unexplored paths
* Moorcraft and Emberlight should have basic healthpods (and 4 of them, please!)
* An "inspect" command to look at other players' gear would be fun! Shows the same frame as your currently equipped gear, only for them.
* Allow scrolling chat with mouse wheel (plus ideally, more customisation)