I remember how many people dislike chapter 970, saying Kaido vs Oden was to short with never seeing both true power. This all can be forgetten if Oden vs Kaido round2 could start in act4.
However, if Oden joins the fight with Kaido, it will be a big nah from me. I don't care about Oda showing us Oden full capabilities here even if, theoretically, I would like to see them. The whole point of this battle and Wano is about the new generation, accomplishing what Oden could not. It's about them being able to bring success where Oden failed. The theme of this new generation is stressed over and over again.
Oden joining the battle and having a part in the defeat of Kaido would invalidate all of that. And I am not even mentioning the other big inconsistencies that would arise if this was to be the case.