Why should players with 'narrow' rosters be matched with other players with 'narrow' rosters only? Why shouldn't they enjoy the benefits of developing their roster this way and also battle players with 'wide' rosters? Why should players with 'wide' rosters have an easier match and only battle other players with 'wide' rosters?
With the current design players with 'narrow' rosters are mixed with players with 'wide rosters' in the pods of 8 in GA and I'm perfectly fine with this. Players with stronger rosters are more likely to win better rewards. I see nothing to fix here.
(No, my own roster is not particularly 'narrow' (3.7 million GP, with a total of 70 g12 and g11characters) , but I enjoy GA and the challenge)