The concern with copyright issues is definitely legitimate, it's all a very fine line between what can be considered parody and plagiarism. On one end, a heavily inspired ninja game clearly on the safe side is Ninja Saga and then you have games clearly not on the safe side and flat on plagiarizing like Naruto Spirit which recently closed down due to the lack of players (because Viz made an official competitor to their game, Naruto Online, a Facebook game)
Nin Online sits on the thin line between the two, we borrow more heavily from the Naruto series with a few monumental locations and the Village Symbols.
Other than that, none of the content is bothered by copyright infringement and trademarks as all the art, music and characters are original.
If we ever do receive a C&D from Viz, we have a backup plan to remove the symbols and a few other changes that will place us on the safe side of the law.
Thanks for all the comments and concerns guys!