We need a new tank Tenacity item
With Mercury's Treads only giving 20% and Zephyr being removed entirely, multi-CC lockdowns are proving kinda brutal at the moment. Yes, you can get that Resolve Mastery that gives you up to +15%, but that's still a multiplicative bonus, so you won't get above 35% unless you're Irelia (whose passive is something Riot hates).
I can understand why Riot reduced the Tenacity on Mercs, though: to reduce its availability to non-tank classes. But tanks didn't get a new tenacity item to compensate, creating the current problem.
To this end, we are in need of a new CC-resist item that only tanks (and maybe juggernauts) would want. IMO, it should be a high MR item that gives you extra Tenacity based on your bonus resists (up to a cap). This way, you have to have other defensive items or a resist-boosting steroid (like Leona's Eclipse) for it to be cost-effective, and thus it can't be poached by other classes that easily. It'd be kinda similar to how Essence Reaver gives you bonus CDR based on crit chance from other sources.
Anyone want to try mathcrafting a suitable template?

and
and 5 points in the swiftness mastery in the resolve tree.