So, you want to make a game like Warcraft 3 or UT2004? Uh, nice intention! And you'll gonna make it alone, in your room, with your super computer? Ah ah ah, you're a funny person!
Sorry, I don't want to be rude! In the meanwhile, check here, please, some tips on level design, some praises to originality and the search to explain why there's so many few good, beautiful and FINISHED Blender Games
Well, this will be the beggining of a Game Level Design Tutorials series by someone who never have done one ( game)!