Losing Darkbomb. Coil with this is meh as fuck. Shadowflame and Hellfire are often needed way earlier, doesn't seem too often you'll have a board worth clearing with a Chogall/SF, maybe if the meta goes super control/lategame. Against aggro decks you either wont' make it this far, can't afford the health loss from the SF, or have already secured a likely win. It is nice on a different target than Chogall himself, so there's some value potential here for sure.
Hellfire already damages you, plus it will damage your Chogall, and you're using it late when you maybe can't afford to lose seven health. Late game HF is often better to save for a finisher anyway.
That basically leaves Siphon Soul and Twisting Nether (or DOOM!...uh, no). Obviously SS is everyone's first "oh cool" reaction, but its somewhat situational to imagine your single SS will be unused, in hand, with Cho ready to go, and have a worthy target.
Twisting Nether, forget it, you're going to blast yourself for eight to destroy your own Chogall with no gain from the body except casting the Nether one turn early?
Barring any new spells, that's all that hand/reno tends to run right? I guess Darkbomb could get replaced by a Shadowbolt, but how often will you have the luxury of saving it for Cho? It will need to be played sooner to survive the storm of aggro/midrange/tempo/combo.
Yeah, ok it could have synergy with Moltens but ideally you might have managed to get one out before then. Its possible that it will work for giant synergy frequently though, I guess.
Honestly I still don't see slow locks surviving the chow/belcher/healbot purge too well. And as others have mentioned there's been speculation that the giants might get nerfs.