[Champion Ult Concept] Swapping health percentages

I thought it'd be neat if there was a Champion (probably on a tank) that had an ult that allowed them to switch their total health percentages with that of another champion, both ally and enemy.

On Ally cast: You want to do this to save your ally. Say the ADC got caught out and is taking damage. You, the tank, with your damage mitigation and likely higher health percentage, can go in and swap your health % with theirs, giving them a few precious seconds of life.

On Enemy cast: You want to cast this on an important target, preferably their carry. Block as much poke as you can, take damage, and use it at the right time in the right place (near their carry) and turn around the fight.

I was thinking the range on it could be somewhat low (maybe around 500 range?), and the health swapping could also give either a speed boost (if they gained a higher health %) or a slow (if they gained a lower health %). It'd be tough to use in teamfights because you wouldnt want to use it on the wrong champion at the wrong time.

Thoughts? :)