Movers with bricks on top, like in RTb 1.045, and buttons that control mover direction to make cars
Unlimited Colors(color wheel or Hex/rgb etc)
Unlimited Addons
Buildable vehicles
Abillity to make bricks easily, and some free shape/line tool to cut bricks and make shapes and things like that
Better avatar options, way more hats and accesories, system that allows people to see your own declas/etc without them being downloaded
Caching or something for saves, so if you crash on some big server you wont have so long ghost, only ghosting for changed/updated bricks
Those are mainly featuers and i doubt an addon can do them
For more possible things:
Modern weapon pack, with complete hand+reload anims, good sights, high quality , lot of weps, vehicles, artillery+ballistic missiles with control GUI
Advanced Nuclear Launchers (submarine,laser targeted bomber,hand launcher,missile silo,mobile vehicle launcher) with proper mushroom cloud, radiation, admin/host limits