I am glad that we can all agree that it would make sense with the lore.
So i guess that balance would be the concern. I can understand why adding the Dragonslayers may be a problem. But maybe if they just added the hunters and zealots? I don't think that they would make Erebor too strong late game, because remember, King Dain must be lvl 10 to summon the Assembly Barrack. So I just think, becuase this meeting is supposed to include all the dwarven realms, Ered Mithrin should be included in that.
I would be glad to here peoples opinion, and thanks to the people who have already replied.