That's not a matter/issue of "enmity" so much as tankbusters themselves at a particular tuning, where that ends up just being a composition check (have tank or rez player). Though, giving all big enemy attacks a means to be collectively mitigated through stacking or lining up in descending order of eHP to protect a marked target --even if that were far less healing-efficient than just using a tank-- would largely deal with that, too.
Enmity exists as it does now, and even as it did back in ARR/HW/StB, primarily to make it so that there's less party gameplay that each member is expected to participate in: Rather than melees swapping in and out, relaxing pressuring and reasserting pressure to allow ranged to briefly kite and then to peel from them as they run out of space, and the party needing to CC, focus fire, and apply timely suppression to specific enemies contextually you get a Blue DPS who prevents the need for any of that largely through just existing. Instead of most of the positional and coordinating aspects of tanking you just have a "Tank" who is free to then act as a simplified DPS with a set of isolated mini-game mechanics atop it.
I'm assuming you mean the T10 mechanic, where damage is cumulatively reduced (and so we tend to line up in %miti order insofar as each sequential member can survive)? I don't think that's the only way to necessitate tanks, but I certainly wouldn't mind seeing more mechanics like that.The only way they can necessitate tank roles without forcing enmity as a gameplay quirk is to just make all raidwides into wild charges.
If there's already good reason to correct the existing state (say, of barrier heals just being superior healers altogether), that your idea would require doing so isn't a significant mark against it.However, this might forces the current heavy mitigation meta into overdrive..