I don't see Wrathion becoming Deathwing 2.0. Deathwing was only corrupted because of the powers he was given as an Aspect; his deep connection to the earth (where the Old Gods were imprisoned). Wrathion does not hold these powers, and while he is still a black dragon, and all black dragons share a connection with the earth, I doubt it's enough for the Old Gods' influence to reach him. I find it more likely that Thrall would be corrupted than he.
Despite all he has done, I think Wrathion is, to all intents and purposes, still a 'good guy' (in the grander scheme of things), and I believe he does have Azeroth's best interests at heart. He may not be flawless, and he certainly isn't as brilliant as he considers himself (I mean, bringing another army to Azeroth to combat its current inhabitants, only so whichever remains would stand no chance to against a larger invasion, what was he thinking?!).
With that said, there is still, of course another alternative. Somehow Gul'dan learned plenty of things about Archmage Khadgar, and he knew that Garrosh was Grommash's son, and while I firmly believe Gul'dan to be resourceful enough to learn this through multiple means (torture being my best guess), it wouldn't be unlike Blizzard to have Wrathion be his source of information in a rushed effort to turn Wrathion into a bad guy..