This theory of mine is actually based on the original ending for the show where Sisko was going to be trapped in the wormhole, but they changed it to the current version where Sisko said he would return because of Avery Brooks' concerns. My opinion is that Sisko said he would return because he was determined to do so but didn't realise that he couldn't. A lot of people disagree with me about that, but I think that Sisko being trapped in the wormhole makes more thematic sense than an ending where he can return at any time, so that's what I go with. And since I don't read the novels, I'm free to do so.
I agree with your theory, I mean with Sisko living in the Celestial Temple, I bet the Prophets will have a whole bunch of stuff to teach him. He would probably only return with the Prophet's blessing or if they send him out on another Emissary mission...