I remember there were "Rip off," items from Gaia (Persona 3, KH, etc.) and I assumed that it was because of copyright issues.
But now there is Adventure time/FMA, which I find weird. If gaia could make avatars based on this, then why not Pokemon, KH, Death note, K-on, The world ends with you, Shin Megami tensei, Air gear, One piece, etc. like what's different now then back then?
To be honest I wouldn't mind seeing Monster gaia items, but that will probably not happen......
I would love sponsors (or knockoff items) for:
Golden Sun (if 4 ever comes out)
Pokemon Black and White 2 (Well, this not as much, new protags have designs that are iffy at best, atrocious at worst. But it would be nice to have some of the other things.)
Star Driver (not gonna happen crying)
Last Exile (Maybe if/when Fam gets an English release?)
You do realize that the people of adventure time and FMA struck a deal with gaia in order for these items to be made. They become sponsors and in return, the deal is basically an advertising deal, they get advertising through said items and forums and the like.
Not everyone under the sun would do such a thing like that, in fact very few would see potential in advertising on here at all.
EDIT: and thus ive said what everyone else already said. One more thing though, a lot of the things you mentioned may not even be legal to distribute in America at all. You have to remember that even FMA was made outside of America and that a company here got the rights to translate and redistribute it.
Pokemon is a similar case, not made in America, its simply being translated and redistributed on American grounds.
A Pokemon sponsor would make my day but that will never happen...
I think sponsor items only stay popular for a while and then the craze dies down, like whats happened with the Adventure Time items. I don't see as many people wearing them as before.