As much as I'd like to see Cartman's dad be Mr. Garrison, I'm afraid my vote for who he actually WILL be is... Kyle's dad.
This would make for some pretty interesting story lines for future episodes.
I know what you're thinking, and no, I didn't see Kyle's dad at the "Drunken Barn Dance" either. But he could have looked a lot different. They all did. (BTW-- LOVED Mr. Garrison's '89 look! That was hilarious!)
Losira