I've been playing a bunch of Trundle Top with Omnistone lately, since he can make use of basically all the Keystones.
I say basically all of them, because Arcane Comet can only be procced by his ultimate and not any of his basic spells. This makes it a dead rune in 95% of all cases you get it, especially pre lvl 6.
This can be very easily fixed. Either change Trundle so his E/Q can proc AC, or use the same system that is being used for Aftershock on AC as well, checking wether you have LEARNED a BASIC ABILITY that can proc it.
This is likely an oversight, due to not having a basic ability that can proc AC being a trait that is possibly unique to Trundle, but it's also very annoying.
Would love to see a fix in the next patch, or maybe even a hotfix.