Idea for a tank item: gives you extra bonus AR/MR while you are crowd-controlled

Since improving Tenacity seems to be something Riot will never do, I think that tanks need to at least be given an item option that gives them extra defenses while they're stunned, rooted, etc. To keep it from being poached by other classes, however, I'd have it be that the item gives you a percentage increase in your bonus AR/MR, not flat amounts. This way, it doesn't do much unless you've already built some defensive items.

It'd work a bit like this:

UNIQUE PASSIVE: Increases your bonus AR/MR by X%. This amount is increased while you are under the effects of a hard CC.

Since it doesn't have any flat value, a Master Yi wouldn't be able to use it unless he decides to go for some weird tank build. But a Leona or Rammus would get great mileage out of it. Juggernauts, on the other hand, would have to sacrifice more damage than they'd like if they want the bonus to be strong.

It doesn't have to be this effect word-for-word, but something along these lines would be a good addition.