There are a lot of powerful bosses and interesting characters in the Phantasy Star games. Which non-player character would you have liked to have join your party in each game? For enemies, they could be like Sari - someone that joins up after you defeat them.
These are my picks:
PSI - Hapsby
I think he could have been outfitted with a laser gun or rocket launcher. Everyone thought he was "useless," so I'm sure he'd welcome a combat upgrade.
PSII - Lutz
If he's so concerned about everything, let him join the party and help set it right himself!
PSIII - ???
Actually, I have no idea here. I guess the most obvious picks are Lune, Alair, or Siren. I can't really see any of them bringing anything new to the play experience, though.
PSIV - The Old Man
While I think some of the Musk Cat hertiage is represented by Rika, I would have liked to see an actual feline join the part in this game. All the other races of Algo were represented directly as a playable character, so it would have been nice to see one of them in the group. If not the Old Man himself, then another Musk Cat would have worked, too.