Jinx; she has an ultimate skin - Star Guardian
lel.. You mean Legendary.
and Annie (Wonderland and technically Hextech as well).
Hextech is technically 1350, as it gives new model/texture, animations, visual effects and sound effects. If it had a new voice over on top of that, then it'd match up with most of the standards of modern Legendary skins. Wonderland, being a legendary doesn't match with that though as it's much much older from before Riot had those standards (and it only changes her Model).
I believe any of the champions can get Ultimates it's just a matter of time for the most part. Though, I also feel that when champions have Legendary skins already, it is a little more unlikely that they'd receive an ultimate. Why? Well, when you look at the 4 Ultimates currently in the game;
Pulsefire Ezreal
Spirt Guard Udyr
D.J. Sona
Elementalist Lux
Each of them has a few things in common.
First, they are all Iconic champions from 2009-2010 eras (beginning of League)
And second: none of them have a Legendary.
So, I believe a champion like Jinx, who was released in 2013, is close to something like that, hence why she recieved her Legendary but wasn't quite there for an Ultimate. Annie or Teemo, both coming from 2009 are some possible choices for that, though Teemo again holds a modern Legendary, so it feels less likely for him.