As long as it's Arkham Origins grade of Deathstroke or something similar.
That said, you guys seem to be really into DC now after IGAU. Coming along, I'd say that someone like Copperhead as was seen in aforementioned AO could maybe be interesting too (bonus point for not being overused and not appearing in IGAU already).
But honestly, I have no bloody idea. I don't even really understand what makes good guest fighter. I would never expect to see Freddy in MK9, yeh he's there, kicking and being awesome.
Or let's take Soul Calibur. Ezio Auditore was looking completely naturally fitting into SC universe, but then there also was Starkiller and Link, of all people.
Guest characters, how do they work?..