Orcish Wieder… Fleshy Undead (German is a hard language to spell!) seems like it must ultimately be a necessity. It was written elsewhere that the various factions all fight each other, meaning that at some point a Necromancer might square off with an Orc Warlord. Since the player could potentially get involved in these fights (I never have, but it seems like it should be possible), then presumably in very rare instances the Necromancer might raise an orc corpse within sight of the player.
Of course, that would probably be really, really rare. So I doubt it would be that high on the priority list.
As for whether or not there should be orcish revenants amongst the regular undead spawns… I have no idea. I’d honestly prefer there not to be, as the two factions are currently very distinct to fight against. Where fighting the undead is mostly a matter of endurance and attrition, fighting Orcs is about being able to deal with their raw offensive power. Having a unit that hybridizes the two would pull the rug out from under both I think, unless it was only found in very rare circumstances.