Dark Knight

[Gamemako]

If they choose an existing two classes then it have to be either Glad and BM or MRD and BM, only options that make kind of since...

From a mechanics standpoint, LNC+MRD would be best for a GLA Dark Knight. Bloodbath and Mercy Stroke from MRD plus Blood for Blood from LNC are easily the most effective and appropriate cross-class skills you can give any Dark Knight. If you start from Marauder, then you would go with LNC and either ARC or THM depending on design. If you are including casted abilities such as a casted stat drain ability, you could make good use of THM's Surecast and Swiftcast abilities. Otherwise, ARC has damage buffs. ACN would be a poor choice due to defensive focus (Virus, Physick, Eye for an Eye).

//EDIT: These being for a DPS Dark Knight, obviously. If they did some sort of stat-draining tank Dark Knight, then the needs would be much more ACN and less LNC.