I honestly don't see how Starcraft could get better.
More units? Sure, a few maybe, but too many and you start getting them mixed up, confused. Nobody new would try it because forming strategies would be too complicated, not to mention the amount of extra buildings and tech tree branches that would have to come with it, along with the spells/abilities that you would have to manage.
Better graphics? Since there are so many units on the screen at any given time, RTS games always have lesser graphics than their RPG/FPS/etc. counterparts. Starcraft was supposed to be a serious, rusted future sort of game, and 2D simply suites that too well. By going 3D you risk ruining the overall image of the game, thought they're already doing that with Ghost...
Extra Races? The same as more units, a few are fine, too many and you complicate the game so that it is no longer fun. If you want to stand a chance against somebody playing a different race than you, you have to know that race inside and out; what counters it, what strengths/weaknesses it has, etc. Too many races and you start to get confused/mix things up.
And lastly, imagine how hard it would be to keep the perfect balance that Stracraft has created. With every unit must come a perectly equal counterpart for every other race. Every new race must be equal to the old races, and so forth.