The problem that Splatoon will face is online. That is, Nintendo's trying hard to make it a multiplayer online shooter game that's fun with up to 8 players playing at the same time, so here's my question to you... Is Nintendo particularly known for stable and reliable multiplayer online games? Let's say Nintendo's servers are up to the task and latency is kept to a minimum, that only fixes one part of the problem. The other is that Nintendo needs more marketshare for anyone to even care to play a multiplayer online game on the Wii U. If the online experience is anything like it is in Smash Bros., which is to say mediocre, then Splatoon will fail. (That's almost a given since we know the single player campaign is an afterthought and the game is designed around the multiplayer experience.)
I've always had flawless internet connection with Mario Kart 8 and Smash Wii U but the problem with Splatoon is that it probably won't have many people online to play with. There's better chances of CoD BO2/Ghosts having more players hooked up for it than a 'kiddy' shooter and this doesn't have anything to do with the fact that CoDs' been out for ages rather simply what Splatoon's going for.
Nintendo should make Shadow Mario a pre-order DLC iniciative because this definitely feels like something out of Sunshine (and Epic Mickey).