SC2 is quite possibly the only game that can't change, but it shouldn't change. You change too much away from what you are, and for every fan you might gain, you'll lose a dozen, because there's such a huge fan base for wanting RTS games to be just the way they are, and the SC franchise has been the gold standard since it hit. You can see a number of examples of RTS games that went off the beaten path listed earlier in the threat here, and while they got points for originality, they ended up not holding anyone's interest for long.
SC2 does what it can in the single player campaign to try and feel a little less like a marathon of maps strung together, offering the illusion of choice and allowing you to play with more powerful upgraded units and mercenaries than you'd get in a typical multiplayer game, but it's the last RTS I'd expect to ever convince anyone that doesn't like RTS games to want to play it. It's not going to do what WoW did with MMOs and create a new market for RTS games, and I imagine that WoW itself sold plenty of copies of SC2 just because you've had faithful Blizzard customers looking forward to whatever they put out.
SC2's basic gameplay is extremely similar to SC, because that's exactly what they wanted it to be, and what most of their customers were clamoring for. I'd compare it to something like Magic the Gathering, which sure, the production values are better and there are more casual play options, but at its core it's still the same old game with very few differences from when it started. There's a hard core audience that demands it, and to do otherwise would be a disastrous move.
