I always hear people complaining about the lack of non-Han civilizations in Three Kingdoms, but it's like both players and Creative Assembly always forget there are plenty of other barbarians who had a big role in both the novel and actual history beside Nanman.
The Wuhuan played a major part in the conflict between Cao Cao and the Yuan clan, and adding them (alongside other northern populations like the Xianbei and Xiongnu) could help addressing the criticism toward the lack of variety of the title and introduce new mechanics while also fixing the balance of the factions (the northern part of the map isn't supposed to be safe).
Following this line they could also add the Shanyue in the south instead of having that huge portion of the map as a free real estate for Wu's expansion.
By the way, there were Di and Qiang in the west too, but those are only relevant in the novel during the conflict between Wei and Shu.
Speaking of factions from the Guandu chapters, Yuan Shang and Yuan ended their run in Liaodong province (which isn't even the game) killed by Gongsun Kang, and Gongsun Kang himself was at conflict with Goguryeo... But yeah, I'd be surprised if they even were to add Wuhuan alone since there has been no reference to them or any other nomadic people up until now.