FMI - ???
Punk Hazard - Law
Dressrosa - Law
Zou - ???
WCI - Sanji
Wano - Momo/Kidd/Zoro
Post Timeskip there is no deuteragonist
PH Momo and Law are shared. Finding Momo is a major aim for the arc, Luffy spends a load of time bonding with him and we start to get pieces of the mystery of him and Kine’mon
Dressrosa obviously Law, but in the first thirty chapters of Dressrosa there is still further setup about how important he is. He’s one of the “cards” they are fighting with Doflamingo over, the Strawhats start to realise there’s some dark stuff in his past and again a big plot point is finding Kanjuro.
Zou is not a question mark, it’s an outright fact that Momo is the second most important character in it. He’s feeling uneasy at the start, there’s the “don’t mention the Samurai” plot, we find out Momo’s true backstory, he makes the formal alliance with Luffy and he controls Zunisha. Law in contrast does nothing.
WCI- neither are in, but at least through Pedro there’s some buildup for the Kozuki awaiting the Dawn and the Strawhats along with the Minks
And Wano, the culmination of all these arcs, is clearly a Momo central one. And most importantly, going forward there’s pretty concrete reason ti believe the Kozuki, meaning Mom, still have a hugely important role to play in the finale since Momo “is not allowed to die” whereas Law is kind of redundant. He’s a less important D than Luffy and Blackbeard (and probably Dragon, and maybe even Shanks). And his new dream is basically just a lesser version of Robin’s dream.
Law’s role as a dueteragonist really revolved around Doffy.