Sejuani passive idea

If Riot thought old Sion was a "mage with an axe", then I don't see how Sejuani is anything other than a "mage with a flail"? I loved old Sion, and I don't like the new rework Sion. In that same vane, I love Sejunai, and I don't want Riot to "rework" her, but she does need some work (like some reduced mana costs).

Her kit is great, but it doesn't exactly fit her role right now; she's expected to tank without any actually tank abilities. Her engage potential is very strong, but also very short range. It's incredibly high risk-reward; if you miss anything you're pretty much dead. Her only real survival comes from her current passive, which is a minor ar/mr and tenacity buff, on hit. Since her Q is not cc immune (and is her primary form of both engage and escape), her passive can be stuffed before it can even happen.

My proposal is to change her passive into a stacking ar/mr and tenacity buff when OUT of combat (it's called Frost Armor after all). I don't know what numbers or scaling would be balanced, but I've got a general idea of what I'd want. Let's say at max level, Sejunai would get 10 ar/mr and 5% tenacity every 1-3 seconds she's out of combat, stacking up maximum of 5 times. So when she engages, she would have a bonus 50 ar/mr and 25% tenacity, and she would lose one stack every time she takes any source of damage (so first attack drops her to 40 and 20%, then 30 and 15%, and so on). This passive would make her engages much less risky, and even give her a little extra survival if she misses (because she would still have her stacks of Frost Armor), but she loses it as the fight goes on, and then cannot regain it until she leaves combat.