So, sejuani, especially right now, is the weakest jungler in the game by far. bad clear, mana problems, her ganks are almost entirely reliant on her landing her q, and if she misses shes in deep doodoo. and lets not forget that her early game is completely void of tankiness and that she is the easiest champ to counterjungle in the game. her biggest problem imo is the lack of tanking abilities. she is one of the only tanks without a shield or a significant damage reduction ability, and it wouldn't be hard at all to put into her build.
yes she has her passive, but im pretty sure that she has the worst passive in the game. it gives her 10-25 bonus armor (depending on lvl) and 10-25% movement speed slowing effect reduction when damaging an opponent with an auto or ability, with the duration stacking with each time damaged from 2 sec up to 8 sec.
this honestly is pretty bad, though I wouldn't think it would be bad if they made it so the buff stacked in addition to the duration but they only stacked twice each. my other thought was converting 0.5% of her bonus hp into flat magic pen. this would help her to deal a little more dmg and it would help define her more as a health tank.
another solution i thought seemed viable was for her w, and this is where my main thought of her tank boost would be. I was thinking they could change her w so that the initial bonus dmg auto, should she land it, grants her a shield for the duration of the ability equal to a bas + some ap scaling + a percent of her max hp.
my last thought was about her e. her e is a mana eating monster, dealing a littttle dmg and slowing pretty well. I don't think its inherently underpowered, but I do think it either should cost a little less mana or the permafrost passive should shred magic resist a little.
this is just a rough idea, but please comment any ideas or notes you have, just as long as you aren't a jerk about it. nobody likes those guys.