Cthun needs one thing to work .. draw.And quest is not really needed as u dont play freeze shaman or stuff.you play a lot of aoes till u get cthun,ig u survive.control shaman sure loves the idea..but keep in mind yogg needs a place in the deck as well. Yogg with quest might be fun af
IDK, I'm Wild only player but in such a heavy control deck you really want Doomsayers or/and Barrels (Depth Charge) for early game. It works in Wild so I expect it has to work in Standard too. I would also consider new Revolve and maybe even Desert Hare for early pressure (turn 4 - devolve theirs, evolve yours - better outcome than with Pit Master btw). Too bad you have no more those Storm Chaser elementals, this card would be great here. But you can add some weapons and draw them with Custodian to thin your deck - Knuckles wouldn't be bad with so many battlecries, also corrupt your 4-drops... just an idea.
Maybe drop some AoE's because there will be always those two cheap AoE spells from C'thun, but you have to test the meta first anyways.
I think running cards like Kragg and Tour Guide would really help this deck. Kragg is fine in any quest deck and with the quest online is pretty strong removal, your plan is just survive until C'thun. I also think something like Licensed Adventurer might be worth to at least test; getting a coin + the decent body is cool and it can also enable the Hero Power + C'thun (if you got 2) or it can enable Tour Guide + Coin + Hero Power + C'thun.