World of Warcraft or Battlefield 2?

WOW is possible, but very unlikely to be done in Gamestudio. For that, you will probobly want (at minimum) the Unreal Engine 2 ($8000 USD). Battlefield 2 could be done in Gamestudio, but the graphics wouldn't be as good. I am currently making a BF2 like game.

For any of that, though, you need to be damned good at scripting.