Problem: DPS winning fights.
Solution: Non-DPS'ing winning fights.
Explanation: A battle where survival ends up winning it. This can be done in two ways. One, where the party surviving for a specific amount of time wins. How they survive doesn't matter, but deaths would increase the time while the enemy would do a lot of off-aggro or AoE attacks, hence it would be healer-centric. Only tank can significantly mitigate damage, so healers role would be more important.
It could also depend on the amount of damage completely negated (by doing a mechanic correctly thus canceling skill, moving out of AoE's properly, using damage-mitigation skills etc.). This would be more tank-centric, as their damage mitigation is superior, but healer would still need to be present to keep the tanks in health, and prevent the DPS from dying. Dying means less dodges, after all. Adds can be used to occupy the DPS's, with adds being normal "DPS wins" mobs, so that high DPS would still be necessary.
These can be mixed and matched, even making a boss that can be won against with either of the three or select two.
Story-wise, it is caused by the boss noticing that no matter how much he tries, he cannot win, hence he surrenders. A reasonable thing and most people don't want a deathmatch just for the sake of having a deathmatch, with no chance of victory...
Problem: Healers and tanks having little reason to go out of their regularly scheduled heals/mitigation skills, as the ratio of DPS to healing/mitigation is more important.
Solution: Make having high health more important.
Explanation: With decrease in HP, comes decrease in damage output. The character gets held back by wounds and the like. Keeping a tank at 50% could cut its damage by 40%, for example, even more at lower levels than that, so "brute-forcing" their way through avoidable attacks would not be a good idea for tanks, while healers doing heavy DPS while keeping the party at low HP until a scripted "nuke" comes would be a horrid idea, as well.
Both of these would be better suited as selectively-used mechanics, for some bosses/mobs, rather wide-spread thing. It would be one thing if the game was made like that from the start, but as it is now, it would cause quite an exodus, I guess. Especially since going by a thread I made in the past, majority of people really don't want to put even a sliver of thought into this game.