Be the logic of starcraft, all would have to be ground units with a non-suicide attack (so they have the ability to capture every other piece and remain alive.)
For zerg I was thinking:
Queen: not sure
King: Kerrigan(or queen if someone thought of a better king)
Knight: lurker (different way of attacking than the other pieces.)
blahblah anyone care to share ideas and zerg/other races.
this thread is dumb, but should be fun.