I highly doubt Dark Link will be a character... he's just in one Zelda game, and only for a second, and he's not even that cool, plus he's exactly the same as Link.
It's more likely to be Tingle (hell yes) if they add another Zelda character, or someone from the more recent Wind Waker / Twilight Princess. I'm personally hoping that instead of "adult Link" and "young link" they include Link from Wind Waker and Link from TP as different characters instead.
Other characters i'd like to see added are K.K Slider (though I doubt it, since he's totally pure man, and wouldn't whore himself out to a fighter) / Tom Nook or someone from Animal Crossing, Wario (a must), and the best thing ever would be to have the bobbing head of Professor Kawashima from Brain Training. Perhaps he can be the end boss as they reveal the master hands were actually his?
The stat whore in me craves more useless stats, I was so glad to hear Sakurai was part of the project for that reason. I want to know how many times i've pressed the A button, how many kilometres i've swung the Rev controller, and with which characters etc. though that isn't essential. Something I really liked about SSBM though was how to unlock some things you had to go really extreme. I want a trophy for everything, I want a trophy for 200 hours game time with just using Mario in a blue costume. A cool idea for trophies I think would be to make it so you collect cartridges of Nintendo games. When you get them you look at the trophy area and it just tells you basic info about the games you've collected (now, if you could actually play them, wouldn't that be sweet? Not realistic though).
New stages. This part has big potential for expansion, with lots of new Nintendo games out recently there could be some neat new levels. A Luigi's Mansion level, a level at Isle Delfino are pretty much shoeins. There's potential for a neat Pikmin garden level. A level where everyone battles on a little island around the Wind Waker ocean? A flippin crazy Wario Ware level?
Items? Let's see, FLUDD and the vacuum could be neat items. I can't be bothered thinking of more right now, but there will no doubt be more Pokemon.
The biggest thing by far, will be how the controls work. I'm not gonna begin to speculate here, but I can't wait to find out how it'll work, hopefully they can seamlessly integrate the new controller to create a fun, accessable and solid experience.