Illaoi's base stats and damages would have to be buffed so hard both because of her extreme reliance on at least two of these items: Black Cleaver/Sterak's Gage/Death's Dance/Trinity Force and because magic damage falls of so quickly right now if this change were to happen. Even then, it sounds like just a shitty version of Maokai without hard CC. You do magic damage, are forced to build tanky because your damage falls off, and you have sustain built into your kit, but you don't have hard CC. Sure, building Rylai's would be a lot more viable than it is currently, but it really wouldn't help her enough to push her into viability.
Illaoi would get behind more often than not due to easily-counterbuilt/counterplayable damage, would offer no CC to the team, would still have trouble getting into the fights as she does now unless the enemy team drastically mispositions their carries, would lack defenses, and would generally be a liability for her team in the majority of cases. This would put her in pre-rework Evelynn-tier, where picking her was widely considered actively trolling.
Illaoi isn't exactly top-performing as it is due to no best keystone option (though patch 8.6 may change this) and making her a magic damage dealer would only worsen her problems.
You do not live as Nagakabouros intends,
-Illaominati