Honestly if Ryokugyu and Fujitora switch sides I think they are more likely to fight Akainu and Kizaru themselves than other characters while Oda makes thematic conflicts about justice in the fights between admirals.
I still don't really believe in the switch sides option though.
It's not like this is a rule but I don't really think Kizaru is a more "serious" character than Jabura even more because of the way he talks. He will likely try to troll his opponent in the middle of the fight.
I don't know but some random guesses :
Maybe Shillew since he's Zoro's opponent or maybe Pizarro since he is a corrupt king.
Maybe Beckman since he's supposedly the strongest yc1 or maybe Yassop if Oda decides Usopp has coc too.