The Duos that I would like to see:
Quan and Ethlyn, maybe for Valentines.
Young Seliph and Sigurd, I wouldn't care the theme.
Masquerade (?) [Sorry I forgot how this theme is called like] Ninian and Eliwood or Nils.
I am not a big fan of Harmonic heroes, but I'll try to come up with something (previously I mentioned that I didn't like them, however, my mind has changed and now I realize that some of them could be interesting):
Canas and Miriel
Lethe and Panne