Wolfox

gender could have possibly be dependent on Corrin  - like it's always been (except for the DLC chapters, were you encounter both once). and seeing that he was Tactician/Grandmaster, the closest class would be Dark Mage into Dark Knight - with Swords and Tomes - maybe would have replaced Odin/Owain in that regard.

Or going into a completely different direction and made him a Cavalier, a Knight or even Archer?

But I guess it would have been to much variables for a "standard" unit to use, basing it of the players corrin (multiple Support convos etc.)

I think it might have been female morgan since male robin is seen as the true robin, and perhaps it could have been a troubadour since strategist