I'd be happy either way.
Mario Paintball would have been nice, using existing characters as they have in every other Mario spinoff franchise, though the ink-travelling/swimming mechanic wouldn't have made much sense. The squid-people look fine and give off a Jet Set Radio vibe, so no complaints from me there.
Personally I don't understand the continual call for new IP's, Splatoon would be the same (i.e. a new shooter game with a fresh twist) regardless of whether it was a Mario spinoff or had completely new characters. It's the gameplay that makes an IP fresh, not your avatar.