It's very likely the next chapter or two will give us some exposition on Greenbull's character, this isn't to say that his role will be underwhelming but at this point in time I don't really see him do anything to the alliance strength wise. Considering how many powerfull adversaries there are for him to face off (Luffy, Zoro, Sanji, Law,Kidd, Marco, etc.).
The way I see it is there's four possibilities:
- Green Bull is ridiculously overpowered and is able to pose a challenge to the Strawhats/Wano. This would be cool, but is extremely unlikely considering it took all three admirals to take on a dying Whitebeard
- Green Bull is another Kuzan/Fujitora situation where he'll let them go free because their good deeds balanced out with their crimes. Maybe in exchange for letting the Strawhats go, Momo will allow the marines access to Wano's resources and help them open up the country safely. (I feel like this is the direction this will go, but I wouldn't mind being wrong)
- Green Bull is secretly against the World Government and is planning to help the Strawhats
- Yamato will beat the hell out of him to prove her value to both the team and the readers