Thoughts on Omnistone, and how to improve
I think that Omnistone is a really cool concept for a rune, but in practice the randomness ends up either making you useless for a little while or giving you just the right combination for there to be little counterplay for the enemy. RNG is never a good aspect to have as a core mechanic in a game like this, and I think all Omnistone needs is a push towards consistency. I have a few ideas that would be implemented **separately **from each other.
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Have Omnistone cycle through the keystones of your secondary Runes So if you take Omni + Resolve, you'll cycle through grasp aftershock and guardian situationally dependent on your ability to CC, and if there is an ally around. If you take Omni + Domination, you'll cycle through electrocute, DH, and predator. You get the jist.
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Instead of rerolling your rune in base, it should reroll when you touch the side of your tower closest to a wall. This allows you to exit lane for a moment to get the next rune, and hopefully have it be one that is more useful in the situation. Slap a 2 minute cooldown on this, unique to each tower.
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When you base, have 3 random keystones show up on the different directions you can walk out of fountain. When you walk over it, you collect it like Rift Herald's drop and the other two disappear. This lets you walk into lane with an optimal keystone, or help you get there faster like with predator. You could also opt to not pick one of these up, and get a random other one whenever your current keystone rerolls. This even allows you to base and then TP with the rune you have not rerolling, which can sometimes be unfortunate.
Those are my thoughts, hope they provide some discussion on the rune! Consensus appears to be that the rune doesn't work well on many champs, and the champs it does work OK on you're mostly taking it to get those sweet sweet inspiration runes.