I'm rather glad they don't have an explicit romance for the WoL—G'raha being glued to the player's hip already chafes my goodwill as it is, and I have a sneaking suspicion the writers would love to hot glue him to the player's backside instead. But if there absolutely had to be canon romance...

Maybe if there was a time skip to put them in their adult years, Alphinaud and/or Alisaie would be interesting. Alphinaud's been with the player the longest overall, and the two of them arguably have the strongest bond with the WoL and aren't, y'know, dead. Otherwise, I'd have to go with... I don't know. My current flavor of the month is Mikoto, honestly. She calls out fight mechanics in Delubrum Reginae, and is also kind of a sweetheart.

Oh, and there's also Feo Ul. Would be nice to retire to Il Mheg when all is said and done, and become their Queen...