I'd probably fix some of the default add-ons that have issues.
Support_LegacyDoors has issues with ghost brick collision, and ifaik Badspot still hasn't fixed some of the screwed up faces in modTer.
Then I'd try to work out some sort of deal with the advanced community coders to maintain the game engine. I do not have the coding expertise necessary to do that.
After that- idk. Work on vehicle collision? The sort of change I have in mind would require all vehicles to be remodeled or reexported. So- I'm not sure if that's even viable. That's alot of game content to throw away for a game update. :<