Simpler building system, not as tedious as now.
Shaders and maps combined, with less cluttered coding so updating can be a breeze.
More blockhead hats, accessories, faces, basically more options to choose from in general.
More default vehicles, like some generic car, and an option to have old vehicle physics for laughs and giggles.
Also, maybe customizable skies (like the system we have now with shaders) combined with the maps, and map skins (like the Grayscale Bedroom could be an option in a GUI)
Gamemodes would still exist, Custom, Freebuild and what not have a map list to choose from, with a customizable Slate still existing.