You could add a sort of hits-taken-linking effect to leech off the MT (or an hp-linking effect to send them your would-be overhealing), but does that not feel... gimmicky, especially if it were made role-wide? At that point, the theme isn't "Your would-be damage empowers me!" but rather just "I charge my aetherodisk with each strike against whomever I've set it to." I could see that working for someone themed around wanting to exploit flat hits against one's party, a sort of retributive damage soft-tank (a Kineticist mage-tank or a dual-wielding Judge, for instance), but otherwise it feels pretty outside the scope of what typically makes for a tank in this game.
But, yes, there is a potential problem in/with OT value that we'd need to resolve. I'd just rather deal with that instead by making MTing itself more eventful and thereby limited (less able to just chug on forever short of massive vulnerability debuffs).
Siphon tanks' excessive passive mitigation towards their maximal offensive throughput. Give them (back) more ways to trade damage for survivability and make finding a greatest (effective) sum between the two a serious part of tank optimization. Finally, just give bosses and mobs enough damage (or, damage events / frequency of meaningful attacks) that a single tank cannot deal with it all. (More specifically, it should cost the party as much in healer damage to solo-tank even in a fight without forced swaps or boss-splits as in trading a DPS for a second tank.) Give us more reason to track our co-tanks' CDs and swap in and out instead of relying just on extreme vulnerability debuffs to force in a second tank in certain fights. OTs then become more valuable, by virtue of their being more reliably necessary in swapping in for their co-tanks survival, AND the tanking itself ends up more interesting.
