Zelda, showcase new IP as a pack in, Mario Kart 8 and or Splatoon. That has to be Nintendo's launch lineup. The potential third party lineup for launch alongside those should make that more than enough for a killer launch lineup without having to blow their load with Mario as well.
That's my thinking too. I think the most important aspect is the new IP. They really need that killer app game that gets all the attention online and in the news to get people to want the system like Wii Sports did. It'll be next to impossible to recreate that level of success, but they definitely need an original title that grabs people and utilizes the system's unique features the most. All of these other IPs are great sellers, but they're all less likely to really entice new buyers like an original game can.
That in mind, I'm leaning towards MK8 over Splatoon coming out first at launch only because MK is a massive seller and super popular. It will help to push units big time even if the new IP pack in doesn't light up the charts. Zelda has the potential but not the track record so they need to have a guarantee big seller. And I don't think all 3 will come out then because you don't want to shoot your wad too early and have your own games compete with one another.
Splatoon would be a great 2nd or early 3rd qtr game. The big Nintendo title for the second wave of games. After that Smash Bros as a lead in to the holiday season with Mario NS as the big holiday game.