Splatoon seems like a non-starter. Mario Party seems likely to have heavy overlap with Wii Party U.
I just don't see much on the table that could really branch out in a big way in 2015.
FWIW, I might be totally naive about this, but I think Splatoon is one of the few chances Nintendo has amongst its upcoming titles to have a breakout title for the system. I suspect it'll either do shockingly well or be another extremely niche title. With proper marketing I think it could have pretty wide appeal. It's an online shooter kids can play.
BTW, I know this sounds a bit silly, but I don't think MP has the appeal WPU does. I think for some reason there's a contigent of gamers that things the WP/WPU brand has a shovelware aspect to it and MP with its brand recognition has more gamer cred. (Which is kind of funny considering they're practically identical.) Not that I expect MP to make any real sales difference for Wii U, though.