Here's a Sejuani thought:
It's her E.
Think of it like this:
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Her Q is a weak knock-up that applies permafrost
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Her W is a weak aoe that applies permafrost
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Her E toggles her permafrost debuff and slows people while doing low damage
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Her R stuns/slows people and applies permafrost
You could just have her W apply an aoe slow around her and make her perma-frost debuff be a damage button rather than a cc button, seeing how most of her kit is cc anyway. Then she'll have something to actually hurt people without needing Liandry's. I mean, I personally agree that tanks shouldn't do high damage without building some damage, but Sejuani does like NO damage unless she builds damage. She needs a button that makes people hurt, so make her W aoe slow and make her E toggle more bonus damage without the slow.
Thoughts?