I see you already have a wordmark and background. I have done something quick, which is not really an improvement in my opinion, but well with Zumo logo, it's not much else that someone can do :)
Like I said, I don't think its much of an improvement, so feel free to discard it. The only strange thing in the current logo is that it contains Kawada's name, but it's not really of an issue.
As for the background. I do have some other ideas, but unfortunately Zumo don't have much clean color images. They are usually full of text (like with the current background) and again it won't be an improvement. The current background is nice, hopefully in the future, better images will appear.
As for the coaches, I'm really behind on reading the manga, like quite behind :) So I can't really answer that, sorry.