the greenbull thing is evident
he won't come to wano to fight, the admirals are subnormal but not stupid, at least not to the point of thinking that a single admiral can face a total alliance, which has just destroyed two yonkou, because they could have sent him as soon as he was heard of the defeat of the emperors, knowing that the entire alliance is devastated or badly injured the heavyweights
no, green bull comes to wano to give news or talk to momo
1: arrives with news about sabo captured, vivi captured or some shocking news for luffy, and thus ask for a hostage exchange (for nico robin perhaps) or for a ceasefire
2: he comes to negotiate with the new shogun of wano and to try to reach an agreement, type marshall plan, they provide food, resources and protection to wano, with some trick as persuasion, after all his shogun is a child
3: least likely, green bull arrives for a direct confrontation against the alliance, but would be denied upon entering the island
4: Green Bull arrives, but he does not arrive alone, he arrives with an ancestral weapon and for the first time (such power or force has never been seen) the world government has one and they will use it as a deterrent effect.