User blog comment:YourPrivateNightmare/Sejuani, the story, the problem, and the possible solution/@comment-24674556-20141104230728

YES! Just yes!

Now with the actual comment. I have to be honest, I don't play ranked and mostly play PvP when I'm teamed up with friends, so I'm not the best guy to give true insight on the usability and nerfs to her damage since her rework. I now that AP or Tanky AP Sejuani got on the spotlight for 15 minutes and proved to be strong, but it's not the "Good Tanky" Sejuani (pure tank with minor AP) the Roit once sold us. Just to end the topic of damage, I'm kinda tired to do the actual math, but they could buff the health% on her W.

But the real part is the "Solution". A real passive. I've once cooked up a different passive for her that has a lot to do with your second propose, Damage Reduction. It would be like:

Shroud of the Northern Winds/Frost Armor/Winter's War Call: Sejuani's abilities and basic attacks apply Frost to damaged enemies. For a N duration, enemies affected with Frost deal X% less damage with their basic attacks (scalling at levels 4/7/10/13/16). This damage reduction is amplified against neutral monsters (either by 50% or 100%).

Then comes another factor: Her E. With you use it, would it keep applying her passive? Can I just spam it and let my W proc my passive again? Well, we kinda need to give the windows of opportunity. So it could go with something like this:

Permaslow: (Just Active) Consumes all Frost debuffs on enemy units(...blablabla). Enemies damaged by Permaslow can't be affected by Frost for a few seconds.

What do you think?