Why I Predict Cho'Gath Will Be The Next Ultimate Skin (And Why He Should Be)
First let's take a look at the criteria for an ultimate skin:
_"An ultimate skin is a complete reimagining of a champion that includes an evolved model and additional features and bonuses (e.g., summoner icons).
Features: new model that evolves into different forms, new textures, new splash image, new animations, new visual, new voice over and sounds, additional/bonus content."
_
So in the past we have had 4 ultimate skins:

An ADC, a Jungler, a Support, and a Midlaner
Logically the next one in the cycle will be a toplaner (which Cho'gath is)
Cho'gath has also had a very strong spot in the meta for a pretty long time. A majority of the season, Cho'gath has been a prominent Solo Queue and Professional Pick. This means Rito will be able to make LOTS of money off of it, seeing as Cho is fairly popular and as far as trends go isn't looking to take a dip in pickrate any time soon.
Cho is also probably one of the ugliest champs in the game. He's up there with
as having some of the shittiest particles and noises around, and a far outdated model to boot. He feels clunky, and he feels pretty awful to play (feedback wise) in comparison to new champs like
and
.
Granting him the Ultimate Skin would basically be a Visual Update. Which he BADLY needs. And since Riot says they aren't working on VU's anymore since that would require too many resources out of other things like reworks and new champs, the ultimate skin would be a great way to bring this outdated champ up to date visually. Example A being
, who's base model is horrendous but who's ultimate skin looks fantastic and almost like a new champ.
And in conclusion, Cho'gath is an evolving champion. He gets progressively bigger, and bigger, and his model requires upgrading. All the past ultimate skins have had some form of evolution or switching between modes -- the colors and particles on Udyr and Sona's, the model progression on Ezreal's, and both on Lux's.
This fits perfectly with Chogath as his model and particles could get better and change as he scales up.
TL;DR - I think Chogath will receive the next ultimate skin because he has a decent and consistent playerbase, his base model needs some serious work, and he evolves like the past ultimate skins