I'd like to add Steam Workshop support in a manner similar to Rimworld's implementation. I'm not sure that counts since Steam Workshop is already on the list, so...
If we're really talking unplanned features I'd like to add the ability to move between maps for solo games it would just load the new map and change your location. For server play, it would transfer you between servers that had some kind of common file system so they could share the character data.
I don't know that I would remove anything, though I would be tempted to either change the water/water source interaction a little bit or revert back to the previous farming -- both of which I do via mods if playing solo.