I hope Zoro's bounty will be robustly better than Killer's but mostly because he should end up defeating King and at least kill Kaido more likely.
The more and more the plot progresses the more I think Zoro wont be much higher than Killer.
With the WG coming Oda will probably payoff Zoro rouding up the samurai and leading them to defend Wano like Ryuuma did all the way back then, I see Luffy, KId and Law being down for the upcoming fight and it will all be up to Zoro, Killer and Momo to defend Wano, so the bounties will go like this :
Luffy 3.3bil
Zoro 1.8bil
Kid. 3.1bil
Killer 1.6bil
Law 2.5bil
With every chapter, my predictions keep changing
Anyway I don't think Killer is done, I don't think he will intervene and fight Queen (what should have happened in the first place) but I think he still has one more fight in him