I feel like Ornn erases his own niche with Brittle

disclaimer: not an ornn player, just an observer

I could care less about his gold acceleration gimmick, it's not an interesting way to play around with numbers like debuffs are. Ornn's brittle mechanic is unique to him and interesting in that it -theoretically- enables him to reduce Tenacity and counter tenacity stacking, which combined with his % damage would make him a passably solid counter to tenacity stack champions, which is unusual for tanks. The problem, then, is sort of two-fold.

A) Ornn can very easily proc it himself, whether through his ult or through his autos. I think the latter is fine, but overall he tends to make the Brittle less about empowering allies and more about empowering himself; an unusually selfish goal for a tank to be having.

B) Ornn already provides so much CC on his own, making the theoretical value of Brittle - making other allied CC stronger to compensate for what should, in a reasonable world, be a champion with relatively mediocre CC themselves - obsolete. So, when Ornn is good, it becomes less about "fit Ornn into the right comp" and more "just pick Ornn," because he offers so much on his own, all in one nice, goat-shaped package.

I know this likely runs contrary to how actual Ornn players and the community at large might feel about the issue, but to me at least, I imagine Ornn would be much better off being narrowed more sharply into the niche that the Brittle mechanic provides. This is just a rough idea of it, but...

W - Damage reduced, both on cast and on Brittle. Cooldown greatly reduced. Unstoppable buff restored. R - No longer procs/reapplies Brittle on the second hit. No longer knocks up; reapplies the slow/applies a stronger slow. Instead, Brittle effects stack and can also stack with W, reducing tenacity/raising the damage on that proc.

This is kind of extreme, and I'm sure there's a better way to go about it, but this would shift Ornn from being a very generalized champion into one whose value as a tank requires proper team-building and coordination, but retains and enhances the vaguely fighter-like qualities that make him enjoyable to play in between the high moments his ult provides. Reducing his general power greatly, and thus his status as a priority pick in professional play, while giving him very clear win goals and team comp synergy.

There's other stuff that I'd like to highlight with Ornn if I could do the work on him, but I think this would be the most sensible, if very ambitious, change you could make to the champion to bring out a proper slot to fit him in among his tank peers and give him a comfortable role to balance him toward.