Defense of the Ancients/WMG

DotA 2 will have a Guest Fighter from another Valve game.
One of the Team Fortress 2 characters most likely.
 * Likely, but this troper would also put his money on a Tank from Left 4 Dead

DotA 2 will be free to play, but have microtransactions, a la League of Legends

 * They'll probably add their store API and have cross-game trading, like they did with Portal 2.
 * A week or so ago, Valve did say they were working on their own free-to-play game while they were talking about how they added MMO Games to Steam. Soon after, they made Team Fortress 2 free to play, but who's to say the game they mentioned wasn't Dota 2?