Categorize the add-ons list better, preview all icons/names for bricks in addon list, preview thumbnails/descriptions for addons in the addon list, add a lot of popular/useful addons as default (duplicator), add some standard GUI stuff (meters all over the place probably)
also load DLLs as part of vanilla and look into better bot pathing
open source it if that's allowed too
oh also a lot of new customization options if possible. more arms/legs/hands/feet, mix & match face decals (so you can have one for left eye, one for right eye, one for mouth, etc. but they're all picking from the same list). maybe hair.