Wii U- I already got one as a gift last year, but if I hadn't, for me to actually buy one, same as OP, Zelda, which I expect in 2015, and hopefully a bit more of X and a new IP. Between Mario Kart, the upcoming Smash, and the promise of Zelda, X, and an interesting new IP, I'd definitely put down the money for it now. Who am I kidding though, so long as Zelda looked like it was going to be as big as the rumors say, that alone would hype me enough to run out and buy one.
PS4- A new Jak might honestly be enough. The IP has been dormant long enough, so start a new trilogy. Buying back Crash Bandicoot would do wonders for convincing me as well, so long as they do it right. I'd wait to see what they did with that first though, whereas for Jak I'd slap down the cash for it out of pure hype. Though there's a lot of things they could do really. A really promising exclusive RPG would probably be the biggest thing. Any exclusive and promising looking IP would help, especially if it does something that seems unique to the PS4. The controller has enough innovation to it now that they should be able to come up with something interesting.
XBO- There's really only one thing they could do, and that's to make a NEW Conker game, directed by Chris Seavor, and designed, written, and voice acted mostly by him as well. The obvious route to go here, for me, would be Conker Goes to Hell, in which they tie up the plothole in CBFD where Berri dies, and even though Conker is a red squirrel than canonically can come back from the dead as much as he wants so long as he has enough tails, he didn't just go make a deal with death to go to hell and bring Berri back with him. The next game should do have him do that, it's obviously very exploitable for laughs and fits in perfectly with the tone and atmosphere of the first game. Aside from Conker, any game more to my tastes (so not gritty shooters) would help, but I'd like to see them do something with the Kinect that really wows me. Something that makes me see the Kinect being bundled as not just a good thing, but a necessity. There was this thing they showed off way back that made use of the Kinect and a projector to extend the game screen over your entire room. That might do the trick.