Alpha Legion Marine, Seneschal DAMN that is brilliant.
I might use that in a serious Black Crusade game
Necron: A Cryptek or Lord would be fine, and I'd actually love to play such a character.
Tyranid: Your best bet is a Genestealer Hybrid. A Zoat might also work. Anything else is severely against canon.
I've converted Dark Heresy to the Only War system. Please take a look!