Give tanks an active item that uses mana to make a shield that scales off of resistances.

I've said this before and gotten upvotes for it, but I want it to get more attention.

League needs an item that can be activated to drain mana to build a shield. The item has an internal cooldown (maybe 40 seconds) The size of the shield scales with resistances, so this shield strongly complements resistances. At the same time, overhaul tank items to have more resistances but less health so that resistances stat-bind to tanks.

This would be healthy for the game on so many levels.
(1) tank power would be gated by mana. Suppose a tank's team decides to dive the tier-1 tower. The tank would be very durable while the mana lasts. The other team might want to disengage. If the tank deactivates his item or keeps it on and goes oom then he will be vulnerable to a re-engage. This is because resistance stacking is weak without a health pool or shields, and a tank using this item and other low-health high-resistance tank items would not have much of a health pool.
(2) tanks would be more durable versus assassins (at least with item activated). This is because resistances counter flat pen and lethality. (3) This item would not be liable to abuse by other classes because other classes would not stack resistances.