Illaoi Skin Concepts
I think the reason why Riot has been struggling to make a new Illaoi skin is because of her tentacles and how they are translucent. This probably makes it hard to make a skin that's not just a recolor. So I tried to come up with a few ideas that (I think) would look nice and be slightly original.
Deep Sea Illaoi Tentacles are bioluminescent. something like http://squid.tepapa.govt.nz/images/gallery/the-deep/article-03/image-01.jpg Idol could some type of deep sea shell.
Haunted Illaoi (Haunted Zyra inspired) Tentacles are spirit/ghost hands (look like Yorick graves) Instead of lashing they smash or scratch. Idol could be a skeleton/undead head.
Star Forged Illaoi My personal favorite. This one may be hard to visualize. The idea is that she was created by Aurelion Sol and looks up to him instead of Nagakabouros. Tentacles resemble Aurelion Sol's body but slightly more translucent. Idol could be a single star (like those of Aurelion Sol's) (Could be legendary with all new lines.)
In case I was wrong and her tentacles don't have to be translucent:
Elderwood/Forest Keeper Illaoi tentacles are vines
Dragon/Serpentborn tentacles are dragon/serpent tails (or heads?)
Master Chef Illaoi tentacles - actual octopus tentacles