A new faction?! No, very very unlikely. I'm around here for years already (not TWC, but FATW in general) and I never heard anything alike. All factions that will be in game, are already included, if only as faction for CBs.
Some have been removed, like the Orcs and (very early already) Khand. There was a map somewhere, showing the territories of all factions and there were but Dunland, Dwarves, RK, Orcs and Elves in Eriador
Also this faction wouldn't have any justification; all the lands between the Misty Mountains and the Lhűn River, Ered Luin respectively belong righteously to the Reunited Kingdom. With the notable exception of the Shire. Even the Breelanders regarded the Kings of Arnor as their sovereign (sort of. As long as there was beer and weed they didn't really care I think
). Dunland's status is not really known, they possibly remained (sort of) independent. A faction placed between Dunland and the RK wouldn't have a chance to stand their ground and would be crushed very soon.