What would it do for Illaoi's issues if...
...tentacles had 4 HP, ranged attacks deal 1 damage, and melee 2?
Also I'm not sure they need to expire as fast as they do, but one thing at a time.
Feel free to spitball something similar for Zyra plants.
Bonus thought while I'm here: Illaoi talks about being in motion a lot, and cc chains ruin her just as fast as anyone else. Juggernauts are supposed to be unstoppable by definition. Why not some small amount of tenacity on ult? Either during its active phase, or a lesser amount as a passive?
(Illaoi's "issues" being that she is super fun against people who don't know what she does but gets increasingly frustrating to play as against people that do, swiftly to the point of uselessness, which is exacerbated considerably when ranged champs get involved.)