I've actually owned Sej since she's first been released, she's just been so underwhelming that I neglected to play her until a few days ago. In my first 3 games as her, I played her in the jungle, as apparently thats the only way to play her. So I've come to the opinion that she's a mess. Her ult is fantastic, but that's about it. Her early clears leave much to be desired and even though she's in the "CC tank" category that Naut, Mumu, and Leona are in, she doesn't do it anywhere near ad well as either of them. I came up with what's (in my opinion) a modest fix to a majority of her problems
Frost: Sej's passive is only decent on it's own for chasing. to improve that, i recommend the slow increase at levels 6/11/16, capping at 25%. To improve Sej's pathetic jungle survivability, I also think that creeps and neutral monsters afflicted with frost should deal 20% less damage. Of course, to avoid being broken, Baron and Dragon would be immune to this
Arctic Assault: Sej's charge, while being a cool gap closer and escape ability, is pretty underwhelming when it comes down it. To repair this, I suggest, firstly, that the hitbox be widened to the width of Sej's boar. Second, the charge should apply in a small knock-back/up similar to Riven's Broken Wings AoE. Lastly, a shield should be applied on activation, Sejuani has no real sustainability, and this would fix this known problem. All of these proposed changes would give charging into a fight a much stronger purpose. These changes would also justify the skill having a ridiculously long cooldown
Northern Winds: The Aoe skill is used primarily for clearing jungle camps and it honestly doesn't do that very well. I suggest the base damage be improved by 10 at all ranks and the empowerment be improved to 75% over 50%. That would allow much faster jungle clears while not being considered overpowered.
Permafrost: One look at this skill, and you instantly know that it isn't actually used for the damage. This is supposed to give Sej's passive purpose, its a moderate slow that turns massive at later levels. while it does its job well, i feel that the range is incredibly lacking at a measly 350. Increasing this to 800 would allow Sej to secure kills for her team by catching fleeing enemy champions. *EDIT: It was pointed out to me that I misread the wiki, the range is actually 1000 on this skill already. Thanks to those who alerted me to this, you know who you are.*
Glacial Prison: I can say this is the one thing that needs absolutely no change at all, it does everything that Sej needs in a team fight. I love what it is right now.