I don't think C'Thun will be competitively viable. I don't see how they could be. A deck full of mostly bad minions that wouldn't even be playable in any other deck, and then a one-of 10-mana win condition that you might not draw, let alone survive in time to play. There's no logical reason why these decks should be any good. Like you can almost mathematically deduce that they won't be. Especially with the entire suite of anti-aggro cards rotating out and not a single one of those cards being replaced by something that's as good as Chow, Deathlord, Healbot, Belcher, Lightbomb, etc. Aggro decks are not losing anywhere near this much power.
A C'Thun deck must, by necessity, consist of at least 30% cards that are literally arena quality. Vanilla-statted minions with no text. The 7-mana legendary is good and a couple of the class cards are as well, but you need to play a number of the bad ones to enable them. And the win condition is a single card, and it's a 10-drop, and you don't have space to run an all-removal/healing deck like Anyfin so that you can actually survive until turn 10.
And then it's not even like C'Thun reads "battlecry: you win." Usually it'll be maybe a 12/12 or 15/15. If the opponent has a board, or just high health, the C'Thun won't kill them. Then what? Most of your deck is devoted to this one-trick pony, and is by definition going to be weaker during the whole game up to that turn because you're playing Spider Tanks and River Crocolisks, and spending a lot of resources cycling cards because you must draw C'Thun by turn 10 or your whole game plan utterly collapses as your win condition is a one-of.
Anyfin decks usually need 2x Anyfin to win because playing a deck like that, with weaker minions that you run because they synergize with your finisher, means even bursting for 20ish won't kill your opponent. C'Thun decks are very similar in that regard, but you can't run two of your win condition. All your opponent needs to do is clear your weak minions every turn. Even if you play Alexstrasza, you have no control over C'Thun's damage and they can have a couple of minions in play that might absorb half of it. C'Thun usually won't be lethal on its own. It's not gonna be 40/40 by turn 10. It's unlikely to even be 20/20. People wrongly assume that he always ends the game.
Then when you don't kill your opponent, because they had an easy time staying ahead against your awful minions (notice how all the C'Thun minions are defensive; how are you going to race your opponent down to low health?), you're sat there watching as your opponent uses the removal spell that he has been saving - it's hardly going to be a surprise C'Thun - and then you lose because your deck was built around C'Thun.
There's no reason such a deck should be good.