I've long advocated for adding tank/heal specs to the pure dps classes, if for no other reason than to ameliorate DPS queue times. I always level my warlock first, but god those wait times to get early dungeons going. -_-
Plus, you know, it'd be something new and interesting. Maybe.
Warlock already has the old Meta tank build, so the groundwork is there even without the transformation itself to keep the DHs happy. Spreading Soul Link across the whole party could be fun, letting the entire party share in the damage
and contribute to the Soul Leech shield. Incentive for everyone to perform well. Mostly I miss AoE drain tanking half the world at once
Could have an automatic sort of GoSac when summoning demons in tank spec; baseline armor boost/raw damage reduction, plus different bonuses based on the demon used. Imp for increased self healing or general healing received (good for solo), Voidwalker for extra physical damage reduction, Felhunter for magic bosses, Succy for more dps throughput or CC potential (also good solo).
For
Rogues, bringing back Combat as a tank spec seems reasonable. Lower CD on Vial/tune it down a bit, turn Feint into a more active mitigation ability. I don't know Rogues all that well to really go into the nitty gritty.
Survival can be
Hunter tank spec or something. I'm not really familiar enough with that toolkit to speculate. But I also wouldn't complain about a healer spec that relied on cute animal snuggles and tail wags.
Mages would make interesting healers. I dream of a Chronomancer spec, healing by reversing damage (a la Alter Time), using shields (they already have personal shields so it's not a stretch, and PW:S could use some company in the toolkit club), quick Armor buffs, in the same vein as Earth Shield (Ice Armor for armor boost, Mage Armor for magic resistance/resource generation which feeds the tank's active mitigation, Molten Armor for thorns-type damage), and a cauterize/fire themed set of regular, direct heals for emergencies.
Or just make 5 mans into 8 mans by adding 3 more dps to the run and boosting mob health a bit, I dunno. I know Mythic is all the hotness these days so premades are where the action is, but sometimes it's nice to just relax a little with some randos... or make my hair go grey. It's a gamble.