You can’t sue anyone over an idea. The only way you’d have a copyright claim is if they used actual content from your game, like if they took models or sounds from your game. You might also have a claim if they used your character designs - which is harder to prove. There are varying degrees of this stuff. You could make a game and call it “Rampage” and be totally safe so long as it wasn’t monsters destroying cities. You 'd also be safe if you made monsters destroying a city that were different from the Rampage monsters, and then called it “Crush Stuff”. The gameplay is not something you can copyright, which is why most modern games are all just ripping each other off or ripping off older games.
There was actually a huge storm in the indie gaming world not too long ago (and it’s still sorta ongoing) about a guy who trademarked the word “Edge”, who then sued an indie dev because he named his game “Edge”. It’s more complex than that, but basically people have been throwing their poop at each other over that lawsuit for months. You can read about it at the TIGSource forums. That thread is over 100 pages, and people are still posting updates and comments in there, so check it out.
So I doubt you’ll ever have a claim on Blizzard for copyright infringement. Even if they were going to copy your gameplay, they’d alter it slightly, alter your character and level designs and make their own engine and models. You might have a case if their game looked incredibly like yours, but many companies have sued each other for similar reasons and almost all of them have failed. The old personal computers were overloaded with 8bit ripoffs and remakes of NES, MS and PCE games. There was a whole cottage industry surrounding ripping off the hottest NES game at the time.
But don’t let that stop you. Street cred is marketable. Getting ripped off by some other company would only make your games that much more popular if you marketed them as “The game Blizzard ripped off” or whatever.