Sorry to do this guys, but I can't help it...
Somehow, I don't think the real question is of Cartman's father, but rather, are they willing to PERMANENTLY change the status quo of the characters?
Remember, this is a cartoon, and events are constantly forgotten (or only remembered in passing). How else do you explain Kenny coming back to life, Kenny's father reappearing after being catapulted against a building, or the Terence and Phillip show being uncancelled?
Even though I do have my belief on who the father is, are they going to reveal it? I don't think they will bother, because it will be quickly forgotten, because doing so will permanently change the status quo of the characters.
But if they do reveal Cartman's father, it won't have a permanent effect. Cartman will still be the obnoxious son-of-a-bastard that we all love and hate, because the father figure will not be a prominent character. I hope.
Leave things the way they are!
Peace, Vince Yim