Season 13 will feature RBG's as being the equivilent of 't2' where big caps will create a massive gear disparity so it's either RBG's or you're going to fall behind, fast.
Specs such as windwalker, enhance, ret, feral, arms, sub, can be used, but they'll never be as good as adding another lock/boomkin etc.
While it isn't class imbalance at fault, but the fundamental differences between ranged and melee. Melee unavoidably end up sitting infinite aoe snares/cc's and swapping targets takes longer.
Problems also extend to the dynamics of certain bg's, for example temple of kotmogu having ranged dps from sides means there's constant dps, and they're closest to new orbs. Range dps can dps/cc whoever they like whilst sitting inside the circles of mine carts in silvershard.
Ultimately the constant buffs to RBG rewards are going to make the gear/rating disparity quite large between viable and non viable bg classes.