I really like this theory, don't listen to the people saying Madara isn't important anymore. He was the most powerful character in the series until Kaguya showed up, and like you said yourself if Black Zetsu can become the most relevant character in one panel, so can Madara.
I think it would be really cool to see Madara revived infront of the Hokage's and Hagoromo. That would be the most epic intervention ever, lol. But the real question is would Madara be revived as an Edo Tensei, or in his living form? Also I don't think Madara will give in easily like you said, he wouldn't turn back on his life's work so easily. Either way I think the intervention would be really cool, and I would be satisfied if Madara became good or stayed evil. If he stays evil, we get to see him die in an epic fight. If he becomes good, it would be really awesome to see him and Hashirama as friends again.
Still though, I think he will stay evil. It just seems out of character for Madara to become good. Or maybe Madara is just a misunderstood soul who is really just lonely and secretly wants to become good, haha.