The only hypothetical benefit I can see to Omnistone is using Electrocute or Aftershock ( or technically Arcane Comet I suppose), and then getting another keystone quicker than the CDs for the aforementioned ones. But since the cycling is random, it'll probably take more than the Electrocute CD (25 seconds) to get another skill that's as impactful, unless you're somehow fighting constantly without killing each other, maybe Maokai vs. Zac.
Aphelios
Nobody knows.
I've tried it a few times on Jhin, it's actually insanely fun especially when it comes in clutch. Here's one example --> https://www.twitch.tv/videos/513335256
Maybe characters that can make use of any keystone, like kennen. If you can reliably trigger the keystones, the short cd is probably decent.
There is just way too much variance for this ever to be balanced. Riot just needs to put in the effort to pick say, 4 appropriate keystones for each champion and have it cycle between them.
Bard: Comet, Electrocute, Hail of Blades, Guardian.
Zyra: Comet, Aery, Dark Harvest, Electrocute.
Trundle: Conquerer, Grasp, PtA, Predator.
Etc.
Its good on anyone that can make use of multiple runes effectively, and doesnt need to specialize in any one.
Illaoi and ezreal are the easy lists, as post buff it will likely be their best rune, as its already very close to being such.
