I can believe the more third party thing, as there are a few Third parties I can see wanting in and agreeing to be in on this:
Square Enix: Due to Itadaki Street DS, Square is an easy in on this. A Dragon Quest or an older Final Fantasy works, Crono or Lucca also would make a good in.
Capcom: Capcom works with anyone, seriously, they love crossovers. Rock Man is an easy in.
Bandai-Namco: Soul Calibur 2 had Link, so someone out of Soul Calibur would be interesting. Klona would be fun too.
Hudson: Bomberman makes our second Man name to be a good fit, but I'm not sure how much Hudson is a fan of crossovers.
Ubisoft: Ubisoft is pretty friendly, but isn't Japan based (which hurts it slightly). Rayman could get in but I don't see it as likely as the BN, Cap, and Squeenix.
More third parties I don't think will hurt if they are all varied as to where they come from. So long as we don't get 4 playable Sonic characters, 3-4 playable MGS characters and such, I'd be happy with everyone getting something. ^_^