There are a lot of late antiquity and early dark ages Eurasian civs they could add since the game is still quite sparse in that period…
Essentials:
Vandals, Nubians, Saxons, Longobards, Gokturks, Hunas, Picts, Syrians (Umayyads), Guptas, Tibetans, Xianbei
Mid importance:
Hepthalithes, Axumites (split from Ethiopians), Alemans, Swabians (Suebi), Arabs (pre Muslim and rashiduns), Alans, Himyarites, Khazars, Avars
Others:
Bavarians, Frisians (they could be added to represent Dutch as Franks represent France), Wends, Rourans (proto Mongols?), Tiele (proto Turks), splitting post Han china and Korean three kingdoms
There are probably many I’m missing cause I don’t know much about the same period in south Asia.
A Visigoth and Ostrogoth split is too much imo, not enough info to justify or make something interesting out of it.