First, I think Sej should be measured against other AoE jungle tanks like Malp, Amumu and Moah. Right now I, and most of the community, think she is a relatively unimpressive pick compared to any of the before mentioned and many others.
Second, her skill set is wonky, redundant, and unimpressive, again personal opinion and, as far as I can tell, general consensus. I think that her passive, Frost, can and should be tied as a scaling passive for E. This gives you the ability to create a new passive, and the opportunity to add another animation (hopefully incorporating her mount!).
Finally, Her skills' long cooldowns and poor scaling make her gearing a lot less diverse than it could be, as currently AP is useless on her and worse she's left with out very much to a lot of the time, and she is easily ignored.
My suggestion would be tie Frost to Permafrost and add a new passive, Let's call it Gore, that did an AoE cleave every fifth attack which would add magic damage and on hit effects such as Frost, now tied to her E. This would give Sej a new gameplay mechanic where, in team fights she would have to time her Fifth attacks with her enemies clustered to quickly apply Permafrost. It would basically give her something to do after she's blown her load and a reason for enemies to avoid her in team fights.
Edit: I think an on hit AoE passive would be really fun for her core gameplay, but I also think it would Open up a lot of fun build paths for such as Ionic Spark and Witts End and buffing her scaling or cooldowns would make Abyssal Scepter and Rod of Ages a lot more attractive too. What I'm saying is that Sej is not a low lying fruit, she will take a little work and a lot of re-balancing but I think you have a lot to work with and can make a truly epic character that feels a lot more reactionary and fun to play.