Why not just make everyone a DPS?

[RapBreon]

You make both mechanics at the same time.

Let's imagine a big boss who create parts of him. Each time one part is killed, he loses a 10% of its life, and you need one tank for the main body and one for the part.
You could have any tank on the main body focused on surviving high damaging moves (So, any tank with high VIT, but "mostly" PLD or DRK, since damage is less relevant), and one tank on the spare part focused on dishing out damage (So, any tank with high STR but mostly WAR since surviving is less relevant)

You could even alter the main target to switch periodically to either physical or magical busters, thus each tank will have its strong and weak points in the long run, and the spare part to take reduced damage from physical or magical so that mixing DPSes will be more efficient.
For an additionnal mechanic, you could make the body to stay in one form (physical or magical) when it hits 50% HP, thus managing your timing so that the final from better suits the main tank you have.

You can do this kind of stuff. And MMOs do. But it's a gimmick, you can't fill an entire tier with it. One boss a tier does not make. It's a good idea though, don't get me wrong.

And in the current FF:HW climate it wouldn't work as is. If the current level of DPS checks were transplanted to your hypothetical, you'd still want a tank who does more damage. Build aggro on head, switch to limbs to help DPS.

Your problem is with the tightness of the checks. As I have said, it is binary. Progression through difficult bosses (mathematically speaking) close to release has always been about squeezing DPS to the maximum (single heal Garrosh world first anyone? What a god).