With the shift in philosophy regarding, bring the class, not necessarily the player, and the addition of more class uniqueness that entails; would this not be an opportune time to perhaps make a serious push for adding warlock to the classes with an tanking option?
Think about it. We are still four pure-classes in the game; two of which are pet classes, three if you count mages with their water elementals. Blizzard just tried it's hand at making both demonology, and survival feel all new-, and fresh, in legion; but what if changing up the damage dealing style isn't the way to go. What if you instead started the hybridization of the pure-classes? According to the data available on worldofwarcraftgraphs, as per Feb 05th 2018, 34,5% (global), and 31,8% (at max level) of the characters played are pure-classes.
Hunters (9,8%/10,7%)
Mages (8.5%/8,7%)
Warlocks (7.1%/7.6%)
Rogues (6.4%/7.5%)
This is about a third of the playerbase, tied to a character who can neither tank-, nor heal. Obviously this is selling point for some players, either because they enjoy the specs currently available, that they might not be asked to take personal responsibility, or any number of other reasons. But now as we are also making the move towards smaller group sizes in organised content with M+, with Island Exploration, and as Ion recently talked about, 6 man rated battlegrounds. Would it not be better to allow these classes to atleast have the option to contribute in other ways then as one of three DD/DPS specs? This would mean that people are less likely to be-, or atleast feel like they are being forced away from their main toons if they and their group of friends needs the occational tank-, or healer.
As for Warlock, we are already known to be the meatier of the casters, and we have an history of filling this particular role already (although in unorthodox mannes); and despite blizzard having already dismissed this idea before, they keep adding in fights that do require caster tanks. Most recently in mythic nighthold. It doesn't even have to be an outright caster tank either, you could have the warlock use fel magics to enhance their physical prowess, again, look to the Gul'dan fight. There should, in my opinion, be enough to go on, to make a lore-friendly, perfectly viable, and not to mention interesting tanking specialization. As for which spec should be up on the chopping block, that's not for me to decide.
Well there is my five cents. Any thoughts?