If I remember correctly, Luna was confirmed to be a guest party member at some point. Who knows if that won't happen with Gentiana and Aranea. And it really makes sense - the main party is a tightly-knit group of guys and this is their journey, the story seems to be about their bond, so other characters (even including Cor) just being guests makes total sense.
Also, when you say "brotagonists" all the time all I picture is like a whole stampede of Yu Narukamis because he's the only character ever that I've seen referred to as a brotagonist
