The argument against Omnistone - a pointless rune.

TL;DR SUMMARY There isn't a situation where you would rather have a random keystone over the best keystone for your character.

I think Omnistone is a badly designed rune that will discourage people from playing well (A trap rune).

Let's go over the problem.

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The Advantage

Omnistone grants a better cooldown on its use than SOME runes.

For Predator, the cooldown goes from 150 to 8 seconds. For Aftershock, the cooldown goes from 35 to 8 seconds. For Electrocute it's 20 to 8. For Arcane Comet it's up to 20, to 8. For Phase Rush it's 15 to 8.

This value changes to 12 for Ranged Champions.

The Disadvantages

However, for all other runes there is no significant importance. For Arcane Comet at a high level, Fleet Footwork, Glacial Augment, Grasp of the Undying, Hail of Blades, Lethal Tempo, and Summon Aery, the cooldown was already lower than 8 seconds and you don't get a shorter cooldown.

It goes without saying that every champion has an ideal Keystone or two. Let's take Nami as an example.

Nami's best Keystone is widely accepted to be Summon Aery. Nami has both healing and damaging spells, and Aery buffs both. In addition, Nami's skill Tidecaller's Blessing synergizes with Ardent Censer while she uses Summon Aery - the buff which normally just boosts autoattacks will also apply a small shield - which in turn will trigger Ardent Censer's attackbuff. It is considered the best because it's highly adaptive, requires no modification to your combat style, and offers a further adaptive benefit if you do switch your items to suit it.

Other suitable Keystones include Arcane Comet, because Nami easily slows and stuns which raises the damage Comet can inflict, Electrocute as Nami can quickly connect 3 sources of damage using slows and stuns, and potentially Phase Rush on a skilled Nami, letting her poke herself into a speed boost to initiate safely.

Nami won't benefit from Aftershock much because she's too squishy to get that close. Predator might be useful for joining battle but she should already be there. Fleet Footwork, Hail of Blades and Lethal Tempo all require a level of aggression that might be dangerous and generally unsuited to Nami. Glacial Augment might work, but she already has skills that apply similar effects and the slows will not stack well. And while Grasp of the Undying might be meaningful on a hyper-aggressive Nami in making her less squishy, the bonus health from using it twice in a game randomly, would be totally meaningless.

Between Summon Aery, Arcane Comet, Electrocute, and maybe Phase Rush, Nami has 4 runes she could potentially use, and 7 runes she shouldn't really consider.

And even assuming she only gets the 4 she wants, her fighting style would need to change to suit her rune. Suddenly getting an unexpected speed boost from Phase Rush could be fatal, steering Nami straight into danger. Needing to combo to inflict an electrocute, then land a slow to hit a Comet, then finally throw a heal while you have Aery - needing to constantly change to match your rune will be inefficient and confusing.

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My Verdict

If you want to have any one of the Keystones more than the others, why ever switch? There's no incentive to do so. The slight cooldown advantage you might get is not worth the likelihood you won't get that Keystone back the next time you need it. You could end up getting Predator, your needed Keystone, then using it to engage combat - getting to use other Keystones while Predator cools down - and then ... have Predator come up at a time you can't use it and watching it expire, or missing an opportunity because you were stuck with Grasp of the Undying.

There isn't a situation where you would rather have a random keystone over the best keystone for your character.

So Omnistone is a pointless keystone and a trap.