Splatoon was stated to be one of 3 prototype games coming from that team. So there are at least two others - assuming they can make them function well enough to be presentable at some point.
That being said, as with any "why is x company not making more new IP games" - it's risky, but can be done and success breeds more willingness to do it again. Splatoon, if the sales back up the internet's love of it (and yes, the internet loves it), is going to encourage Nintendo to take more risks with new IP. I mean, we're focusing on Splatoon, but we can't ignore they also took a risk with Code Name S.T.E.A.M. which flopped (sadly, it's actually not that bad of a game). Nintendo seems to be getting a more "back to roots" approach starting up with the young team doing their thing. They seemed to remember that at one point, Nintendo's very best were just a young team trying out new ideas that became Zelda, Mario, etc.
Keep the current franchises going strong, bring in new fresh IP from younger developers with new ideas, and Nintendo has a recipe for success long haul again. Really hoping Splatoon sold well in Europe and NA.
I also love how they approached Splatoon. It was a low budget game just put out there - no big marketing budget initially - just to see how media would react last E3. Media went crazy for it because it was just so much fun. That buzz, coincided with some fans expressing it, ultimately lead to Nintendo's willingness to give the game a big marketing budget, at least in NA. It's almost as if... they let the game earn that extra budget for marketing instead of just giving it to the game and watching it flop. Conversely, reaction to S.T.E.A.M. was tepid, and thus it didn't get the marketing backing.
