Hi! I've been enjoying this mod greatly in the past weeks, and while there's already an impressive amount of classes, I feel like Death Knight would fit in perfectly.

Now, I know there's already the Dark Templar, Necromancer etc. that are very similar thematically, I think Death Knight could be made something unique with a similar "stance" tech that many of your other specializations have. Personally, I think there are many good ways to make a thematically, and practically, enjoyable Death Knight, but as an avid world of warcraft player I came up with a Death Knight idea centering around 3 different "elements" (or specializations, as they are conveniently called in wow): Blood, Unholy and Frost.

There could be 3 stances, each having its own bonuses and quite possibly an activatable ability. Blood would be centered on tanking and self-healing (through leeching, bleeding etc.), with iconic abilites like Blood Boil, that boils the blood of nearby enemies and could apply a bleed to the enemies.
Unholy is centered around summoning the undead, disease damage, and possibly unholy magic like Death Coils (bolts of unholy power)
Frost would be centered around doing direct damage, obliterating the enemy with swift weapon strikes, and slowing/freezing enemies with iconic abilities such as "howling blast", doing aoe frost damage, or a self-centered icy storm that freezes and damages enemies.

As far as possible weapons go, I can't help but take examples out of wow again: if it's within the limits of possibilites when making a mod, a weapon that changes form would be the Death Knight's specialization weapon. 2-handed for Blood and Unholy, and splitting into 2 one-handed swords (or axes, the idea is the important part) for Frost. Maybe it could look the same for Blood and Unholy, but have a different color variation depending on which stance you are. Red for Blood, Green for Unholy and Blue for Frost.

If you like the idea and would see it go further, I can help you with ideas, balancing and possibly some direction with weapon sprites.