Well, first thing first.
Who of the Topi Roppo is going to get the chance to challenge one of the All Stars?
Well, the thing is, is that I don't think that they will get the chance to challenge all stars, and there are multiple reasons for that.
1. First reason is that there is no point for that fight, Oda doesn't have the time to draw a fight for example between "Who's Who vs Jack" and even if he had, there's still no reason because we know who'd win, on top of that the fight like that would injure one of the all stars which would nerfe him for the fight against the strowhats and I don't think that Oda would do so, not with all stars at least.
2. With all do respect I don't think that the flying six (even the strongest ones) are camander level, I know that people have high expectations form beast pirates but there's no way... just no way that Oda, after building up all the calamities so long will replace them before the final fight, it just doesn't have sense for me.
3. The flying six are mostly introduced as a stepping stone for the strowhats, particularly Who's Who vs Zoro and Susaki vs Sanji, and all this teasing with them having a chance to challenge the all stars is way of hyping them up and give the reader some understanding of their power level.
Or will it be someone else?
Zoro getting lost again will his way lead him to Yamato?
Will Luffy find Yamato and make him an ally?
Or will it be Kidd, first blood for his Revenge against Kaidou?
In order to answer that question we first need to understand Ymatao's purpose in the story. Beasd on the fact that Oda introduced him so late in the story, when we in the middle of the arc or even past it, means that Yamato can have only three purposes for this arc.
1. The first one is that he's a child around the same age as Momonoske so that Momonoske could have an oponnent to fight against in this arc. Luffy is playing a father figure for Momonoske and is going to defeat Kaido instead of him, Zoro on the other hand is playing an older brother figure for Momonoske and is going to defeat/kill Orochi, then raises a question... against whom Momonoske is going to fight and defeat in this arc? It would have a lot of sense for me if Momonoske defeated Kaido's son, since Momonoske is just a Kid and he can't fight against adults. Momonoske fills unimportant/useless since he can't do anything to avenge his father, for Momonoske defeating Kaido's son would be quite an achievement since when they grew up they're going to be close in strength to their fathers.
2. Yamato is a stepping stone for someone (probably for Luffy) the reason I think it's Luffy since he needs the power more than anyone else. Luffy is going against Kaido (a Yonko) and judging by his bounty, reputation, title he might very well be the strongest character in one piece so far. Regardless whether you think Kaido is going to be defeated in "team fight or in 1v1" Luffy needs an extreme amount of strength to do so.
3. He's a wedding plot along with other things but mostly a wedding plot.