My guess? Mario vs. Luigi or Mario vs. Bowser to promote Super Mario Maker. They're extremely recognizable, and Nintendo doesn't have to do any more legal work than normal.
Even if it isn't, I guarantee, somewhere down the line, there will be a Splatfest themed on Nintendo stuff. For instance, the next time two Pokémon versions are released at once, I'd be surprised if there wasn't a Splatfest with each team representing each version.