There are a lot of pirates in other kingdoms.
I think kingdom members should have something unique, but nothing really desirable. Perhaps a disadvantage would be nice.
For example, Samurai have .5 less hearts than anyone else. Yeah.
Pirates can only carry 100 gralat at any time.
That way, every kingdom is unique, but people won't want to be in more than one because the disadvantages will add up.
Yeah, I know this idea will be shot down.