I'm thinking I'll do an article about each of the classes to talk about what we can expect to see and what we'd like to see for the various classes when Legion comes out. We're still pre-Alpha at this point, but that's the best point to think about the direction the class should be going. Some of these articles are going to be in much greater detail than others. I've been somewhat less alt-heavy in Warlords than previous expansions, so some of the guys are going to be a little bit light. But this isn't purely about mechanics, it's also about lore and how that relates to the new class-centric features of the upcoming expansion.
The Knights of Death:
There's a good chance that Acherus will be moving to the Broken Isles - in all likelihood, this will be just like Dalaran, in that the old Acherus will still hover over Eastern Plaguelands, but there will be a new one that truly serves as the class hall. We can expect a re-textured and possibly re-structured version of the Ebon Hold - at the very least, we'll have some facility for dealing with Champions and customizing our weapons. Obviously, Death Knights have always had Runeforges, so we might just get an extra special one for the artifacts.
Given the class-centric nature of the expansion and this feature, it seems like the perfect opportunity to rescue Koltira Deathweaver from the bowels of the Undercity. Still, this should present some interesting problems for Horde Death Knights, especially Forsaken ones - if, as one might expect, you have to fight through Sylvanas' forces to get there, how to you reconcile that with the fact that Sylvanas is one of your faction leaders? While you'll certainly not be able to break away from the Horde, this does seem like a case of split loyalties.
Thassarian and Koltira seem like obvious choices for champions. Darion Mograine's fate is unknown - if you manage to found a new Death Knight order, he might simply step aside and let you lead, but if it's the true Knights of the Ebon Blade that you come to lead, one might worry about him (to the extent that Death Knights worry about each other - well, those who aren't Thassarian.)
While they're quite different sorts of things, it might be kind of interesting to look into the Gorefiend-class Death Knights - perhaps we could recruit some old Orc Warlock in the body of a Human Knight.
Unfortunately, there really hasn't been a ton of Death Knight lore since Cataclysm's Western Plaguelands, and there's not a huge cast of recognizable characters. There is that one Tauren guy whose name I forget, and Crok Scourgebane, but Thassarian and Koltira are clearly the biggest names the class has.
It's likely we've seen all three artifact weapon sets - only the Unholy one, Souleater, remains unconfirmed, though the art for it has been displayed, and it seems highly likely that it's the one. The real question is whether Blood and Unholy will have satisfying backstories for theirs - clearly Frost gets the most enviable one (even if some of us really wish that we could reforge Frostmourne into its old, two-handed version.)
Blood has been a bit weird in Warlords of Draenor, and I think the main culprit is the lack of melee strikes. With both Rune Strike and Heart Strike no longer part of the spec, Death Strike has become literally the only physical ability the spec has - everything else being some kind of shadow-damage burst. While being able to spam Blood Boil is a nice way to keep AoE threat everywhere, it just seems unnatural for a Death Knight not to hit things with their weapon. Giving back Heart Strike as a Keg-Smash-like AoE melee strike (though presumably not as big a portion of our damage) might be a way to fix this.
Diseases are also a bit funny for Blood - being able to maintain them constantly through our regular rotation (unless one goes for Necrotic Plague) is certainly convenient, but it almost makes them feel passive. I honestly don't know what I'd change here - it might just be the perk of playing Blood that you don't really have to think too much about diseases so that you can concentrate on survival.
DKs are pretty solid on the active mitigation front. The change to Rune Tap as a kind of mini-Icebound Fortitude gives us a frequently-available oh-crap button that isn't a regular part of our rotation the way that Death Strike is (you could compare it with Monks' Elusive Brew or Paladins' Word of Glory - to be used liberally, but not regularly.) And of course, ever since Wrath, we've had a wide selection of more serious cooldowns: IBF, Vampiric Blood, Dancing Rune Weapon, kind of Bone Shield, Anti-Magic Shell. Things have to be going pretty badly for you to feel like you don't have any options, though one could make the argument that with so many cooldowns, they're all kind of watered down - when the feces hits the propeller, as it were, sometimes it doesn't really feel like enough to just hit one of these. Perhaps slimming down the options but buffing their power would help (and certainly clear up some space on action bars.)
Finally, ever since 4.0, many (myself included,) have sorely missed Blood DPS. Given the relative success (I think) of Gladiator's Resolve, plus the fact that with the changes coming to Frost (see below) we won't have a two-handed pet-less DPS spec, it seems like an obvious choice to me to give us an option for Blood DPS. Indeed, focusing on big physical strikes, you could give a lot of old-schoolers who miss Blood DPS and potentially less old-schoolers who love the current version of Two-handed Frost a great new gameplay option.
Frost is obviously going through big changes - or at least it is for the portion of players who use two-handed weapons. The real question is whether we're going to get any significant changes to the dual-wielding playstyle. On one hand, I'd prefer the spec to play more like 2H frost, even if I do so with two weapons, but there are also a lot of players who are just used to the dual-wield style. The real question is which style Blizzard prefers - do they like Obliterate to be the massive strike at the core of the spec, or do they want it to be a filler ability to use in between Howling Blasts and Frost Strikes. I'd argue that the 2H rotation makes better use of the available runes, and that once there's only one style for the spec, it'll be much easier to balance it around its Mastery and such so that it doesn't fall behind in AoE like 2H does currently.
In truth, I think Blizzard should have never made the two sub-specs play differently. Fury, as the obvious point of comparison, might have a different rhythm to it, but fundamentally, the same abilities take the same priority.
But given that they're now slimming it down to a single style, they've got to make the right choice for the spec.
With Frost going pure-dual-wield, Unholy will be the DPS spec that uses a big old weapon (unless Blood gets its Gladiator equivalent.) First, as a disclaimer, I don't play Unholy regularly (I have an UH DK Horde-side who's been sitting at level 90 for several months,) but as I recall, Scourge Strike used to be a two-rune strike, and is now a single-rune one. That means that it naturally has to do less damage. Unholy already splits damage with its ghoul, so re-imagining the spec as the "big numbers" style for DKs might not make all that much sense.
Given that Unholy is the only spec that deals with undead minions regularly anymore, it might be fun to lean into that identity - right now there's the Ghoul and the Gargoyle as a cool-down, but I wonder if it would be fun for the spec, and if it would be a direction Unholy players would enjoy, if more minions were worked into the gameplay.
Death Knights don't really have differentiated talents right now, but I think a cool one for Unholy would be to either have something like multiple minions constantly or perhaps to replace the Ghoul with an Abomination.
The thing that's exciting about Legion is that every class is going to get a bit of lore by necessity. But probably the most important absent player - Lich King Bolvar Fordragon - has not made a peep since the end of Wrath. This would be a great time for him to use the Scourge against the invasion, but we really don't know whether the Scourge can be used as a force for good, or if ultimately it's just a matter of time before Bolvar loses control (over them or himself) and we find ourselves battling the army of the dead once more.