I think it is indeed a bit early to be guessing about Urahara's bankai. We're barely even seen his shikai. Other than the blood-mist shield and the powerful blast he can shoot out, we don't know much about his zanpakutou. Granted, he seems to extremely powerful, but even then, I'm not sure Urahara has even revealed all of his shikai. But maybe he has. I don't know.
But I'm really anxious to find out what Urahara's bankai is. Just what does he mean that his bankai isn't meant for helping others gain power? (or something like that...) I'm curious.
But there are a lot of good guesses. I wish KT would tell...soon!