Purely a hypothetical and curious question about other people's thoughts on this.
If we zoom back into the first tier of content in any expansion and have one player who is from the last tier of content in that expansion, which role would be the best to have to make that older content so much easier due to their inflated gear and stats?
An example would be -
In Cataclysm, we had Bastion of Twilight, Throne of the Four Winds and Blackwing Descent, now in a 10m raid for example, 9/10 people are just heroic dungeon geared about to set foot into these new instances for the first time. Another player, be it a tank, healer or dps, is geared in heroic DS gear from 'time-travelling' I suppose (As I said, purely a hypothetical question.) would be the most powerful role to make going through the content easiest?
Tanks would take far less damage and be able to hold aggro with ease with their better gear making it easier on the healers.
Healers would have much less trouble with mana regen, be able to focus more on some off-dps and most damaging mechanics of a fight could simply be botched due to the healer overpowering that with their gear.
Dps would speed up the fight immensely, meeting the enrage would be less trouble, adds would die quickly making it easier on the healers.
I realise the previous tiers had 25m as the top tier and vanilla being 40m, so to make it 'fair' use 2 players of one role in 25m and 4 players from the future content in 40m.
Personally, I would find this question quite tough to answer, but in the end, I would probably guess healers, because good healers can usually mean that as long as everyone understands the mechanics, the bosses will fall with general ease.
I hope I made that as clear as I could, as I said, just a totally hypothetical and curious question for discussion.