Looks like I'm inbetween my Unholy DK and my Shadow Priest for Legion. I've narrowed it down to those two. This is with the intent of fully clearing everything on mythic and finding a raiding guild to do it with. Not looking to be top 100 or whatever, but I'm in it to get everything done reasonably before the next tier of content. With my new job I'll be able to regularly dedicate myself to raiding so it'll likely be a 3 day group that I'll hunt for so we get it done sooner.
UH DK is what I played fairly well in Mythic HFC. From what I've read on Legion, they continue to have most of their strengths and then some. Extremely strong AOE damage, survivability and a death grip(x2, but minus a Gorefiend's grasp). DKs are extremely popular and Melee will be likely evne more crowded in this expansion due to Demon Hunters and the new animations, but DKs have always been popular and yet they seem to be one of the most in demand melee DPS due to their utility and AOE damage. Their AOE is also apparently very good for Mythic dungeons. And I don't mind any of their offspecs. A very cool artifact weapon with a lot of style.
Shadow is one of my favorite specs and has been since BC though I've never gotten to regularly play it. They seem very strong at the moment, but as a former Elemetnal Shaman, I know Shadow Priests damage can swing on a dime in a similar way. I enjoy their rotation, I love their lore, their artifact weapon is cool even if some of the traits aren't that interesting, their talent tree is very cool, and the visuals are awesome. Old God lore and Twilight's Hammer stuff has always been some of the most interesting stuff in WoW to me. Their single target DPS is very good but they aren't as fun to play in dungeons outside of boss fights due to their extremely poor AOE. They are a hybrid with two healing specs though they are ranged which I may find getting into a good guild easier with. But I strongly dislike healing and would never do it. Selfish or not, I do still play to have fun even when progression minded. Just one of the few things in the game I'm active about not wanting to do. I managed through all of Mythic Siege of Orgrimmar as Elemental without once having to heal.
Whatever's not the 'main' will be the alt but with how artifact power works, I don't know how much time I will dedicate to the alt.