To be honest for some reason I feel that GW will goes to 6 factions per grand alliance (+1 big faction like CoS).
Order: SCE, Umbra+DoK, Realmlords+Deepkins, Sylv+Kurnoti, Duardins, Gholemkins, OrderOfAzyr
Chaos: Skaventide, Khorn, Nurgle, Slaanesh, Tzeench, StD(chaos beastmens could be part of God forces)
Destruction: Orruks, Gloomspite, Ogors, Gargants, New gitz, Beastmens(or silent people)
Death: Gravelords, NH, FEC, BoneReapers, +2 new factions.
This will give us 24 factions in total.