Blizzard probably won't care if you download the client and use their map editor. You won't get access to the game or be able to play any maps without an account. Kinda worthless to play AI over and over again.
I'm looking at the tools that we have now and thinking about some things. I do a lot of scripting for another game, and looking at the map editor I really feel like I can re-create actual games in this editor.
Supreme Commander 1/2 would probably be the easiest given it's so similar to starcraft. I need to figure out how to create water-only units for boats.
After viewing some videos on how to do mouse look and attack I feel like I could do a diablo2 style game.
Sidescrolling games are possible. Like mario or something. Maybe some marine 2d side scroller. Need to figure out "jumping".
Do we know anything yet regarding copyright? Could I for example create a Command and Conquer 4 mod that plays almost exactly like CnC4 and looks similar if not exactly like it? Or maybe a supreme commander 2 mod (which I am actually thinking about).
What control does Blizzard have over what is played on their network? I'm not saying I would... but I know war3 had some "porn" maps. Is blizzard going to censor this stuff?