We will need Dain for sure, as he is king-under-the-Mountain, and we wanted Balin for Moria aspiration reasons. There will of course be other dwarves present in the Frame, but we are not likely to 'introduce' them or have them be named characters. I'm not saying there's no opportunity for a glimpse of Gloin, just that we wouldn't be likely to cast him for the role at this juncture. The Frame is ~5 minutes of screen time per episode, and thus over the course of the season, we accumulate enough for a single episode. So, when you think of named characters with dialogue in a one hour episode...you don't want to go too overboard with the Frame. The fact that we already have Thranduil, Bard, Dain, Gandalf, Bilbo, Balin, Celeborn's sister, and possibly Bard's son Bain and maybe even Legolas means...we're getting pretty full.