I really think SC4 is a great candidate for one of the games. Numbers already show the interest in the game. Now that the 'lolCalibur' and initial paranoia of SC4 being bad game or a repeat of console 3 are finally dying down, and people are realizing how good 4 turned out, you're seeing people take it more seriously now too.
Tekken is in limbo with 6 not being on console yet, much less BR. VF isn't in its current iteration either, and unfortunately, won't pull big enough numbers. I know the Tekken community is trying to rally up interest in a TTT tournament, which I think is dope, but don't see fit for a main game over SC this year for the grand finals/content on EVO DVD. That really only leaves SC for the 3D category.
Also, as a heads up, there are nationals for SC4 in July Mr. Wizzard. Not sure if that date/tourney will affect turnout for EVO or not. I'm guessing not for the most part. Most players want to rep big at EVO, and I'm sure both SC vets and new players would want to rep SC there this year.
Outside of the nationals date, the only other issues I would see in choosing SC4 is console choice, and ruleset. Console choice will likely be whatever console SF4 is decided to play on. Ruleset, may be tougher, but nothing like what Brawl has gone through. Currently the community is fine with roundcount/which stages are allowed and all that jazz. The only split is on the allowance of some characters. I'm sure it can/will be worked out before Summer though.