Ah, so many choices... I think I would want Ike as my mirage. Ike is my favourite Fire Emblem character of them all, we do share some character traits (blunt but honest and caring), I would be learning swordsmanship from the greatest swordsman of all time, and it would be really cool to wield the sword Ragnell (or at least the sword I would wield based off of Ragnell).
Alternate choices include the following, but not in order: Micaiah (light magic), Rhys (again light magic and also my favourite healer unit), Ogma (if I have to pick a Shadow Dragon/Awakening character), or Radd (swordsman, Shadow Dragon/Awakening, and a redhead like me).
Going back to Ike, I wonder what Ike would look like if he appeared as a mirage. I'm thinking a cool blend of his unpromoted and promoted outfits from Path of Radiance and Radiant Dawn (armour mainly Vanguard outfit, build mainly Path of Radiance), with a bit of what they did to Chrom when they made him a mirage. Sad that it looks like Ike won't be in the game...