They should bring the Sassanids back. They survived in Western Tang for at least a century. It's possible that the family was still around in 867.
P.S. Why does everyone call them Sasanians these days? Sasan sounds like a Russian circus midget.
They are in game, as a cadet branch. The Bavandids are descended from them, but had broken off before the Imperial line went extinct iirc.
From wikipedia:
The dynasty itself traced its descent back to Bav, who was alleged to be a grandson of the Sasanian prince Kawus, son of the shah Kavadh I (ruled 488–531), who supposedly fled to Tabaristan from the Muslim conquest of Persia. He rallied the locals around him, repelled the first Arab attacks, and reigned for fifteen years until he was murdered by a certain Valash, who ruled the country for eight years. Bav's son, Sohrab or Sorkab (Surkhab I), established himself at Perim on the eastern mountain ranges of Tabaristan, which thereafter became the family's domain.[1][2] The scholar J. Marquart, however, proposed an alternative identification of the legendary Bav with a late-6th-century Zoroastrian priest ("magian") from Ray.[1][3] P. Pourshariati, in her re-examination of late Sasanian history, asserts that this Bav is a conflation of several members of the powerful House of Ispahbudhan: Bawi, his grandson Vistahm and his great-nephew Farrukhzad.[4] She also reconstructs the events of the middle 7th century as a civil war between two rival clans, the Ispahbudhan and Valash's House of Karen, before the Dabuyid Farrukhan the Great conquered Tabaristan and subdued the various local leaders to vassalage. The Dabuyid house then ruled Tabaristan until the Abbasids subdued the region in 760.