With regards to themes: making new themes is something we've discussed, but hasn't been at the top of the priority list as of yet.
Will there be themes for other series?
This is something we haven't thought of, mainly because our branding is as a One Piece forum. I think if we release other themes they'll be One Piece related to begin with, but we'll see. It's mainly up to our designers
-Banner making contests?
I'd be up for this, actually - saves me having to contact DA artists at the very least, hahaha