emnity idea would work, allowing cures to exceed the current cap, dissallowing any DD tanking (since damage done by any source would be unable to exceed the old cap and mages curing you would be killed) and giving pld a JA that allows it to also exceed the cap (give it to nin too but using this JA would greatly nerf its DD capability equalising it to pld as a tank). So like, any form of damage can go upto the current 100% cap at best, but cures, pld&nin using the special JA can take it upto 150%. Every job in the game should not be able to cap hate as they currently can.
Blu and rdm would still be able to tank better than PLD w/ this set up.

Another way is to make new endgame mobs that simply don't take damage well at all under any circumstances and have lots of conal aoe moves(or normal moves being conal aoe, like iron giants), very high -damage taken and -magic dmg (lower hp to make the fight lengths about the same). In this way holding hate via damage simply won't work and again cures would exceed a DD's ability to tank anything at all. Specific tanks would be the only jobs able to hold hate while DD pick away at it. Mobs like this of course would also need to go into hyper rage mode doing massive unstunnable aoes if their hp bar drops too quick (to stop zergs and make the fight more tactical).
Same thing applies, a rdm or blu would be a better tank here than PLD

Lastly make all endgame mobs have high regain and lots of aoe moves, also make them rage within a fairly short amount of time to force you to put a lot of DD on it. Now give pld spells which give defferent types of auras, ie -50%+ damage taken to everyone within 10 distance of the pld. Pld with -dmg taken aura that covers all the DD allows you to take the mob down with lots of DD on it, also the mobs should have regular stages where it cannot be damaged every 5-10% hp taken aswell as -dmg to ranged attacks to stop zerging or getting around the need for auras.
DT caps at 50% anyways, any decent geared DD would hardly be affected by this, if at all.

All horrible ideas.