Someone on the AoS thread on /tg/ mention that we'd see a 'grot soup' battletome, followed by a Moonclan standalone tome - in the same vein as Legions of Nagash followed by Nighthaunt books. No source but conceivable - it'd give Gitmob and Spriderfang some extra utility while still allowing for a centipede heavy Moonclan.
To the same point, I wouldn't be surprised if a lot of those micro-factions were folded into one Free Peoples tome. GW seems unwilling to incorporate models that overly impede on the off-the-wall aesthetics of the new factions, and if the light/shadow aelfs end up being as bonkers as we expect I can't imagine any space left for a lot of factions otherwise. A combined 'Free Cities' tome, or something, bringing aelfs, free guild, dispossessed, collegiate arcane, ironweld, etc. would be a good shout.