The only viable way to play Cho right now is as an AP assassin

This has actually been his primary playstyle for entire seasons now. Not tank, tank Cho's season 1 kit is a joke, but full AP. So unless you plan on completely reworking Cho's ratios, changing Cho's ult scaling from AP to 12% bonus HP is going to kill him. He's not in any sense of the word a reliable fat guy, he lacks the hyper tankiness (no shields, no %dmg reduct, mage level base stats) and engage (0 mobility) of the top tier tanks and the dueling potential of degenerate scum like Fiora. Nerfing almost everything about Cho's ultimate just so you can give him some stupid gimmick where he can get OMGURZZ INFINITE STACKS (cause you're totally going to be rolling in those execution kills with your ult) really just seems like you, riot, dicking the 5 Cho players left around cause why not, not like we have anything better to do LMAO.

So either give him a real rework or leave him in the dumpster where you left him till you have the time to give him a real rework and stop fucking around.