Xianbei
Xianbei is like the Goths of East Asia and can barely cause any controversy. Xianbei immerged into Han Chinese, Mongols, Turks and Khitans. Like Goths they appeared at Han border, fought against or along with Han empire, joined the warlords for unifying China during chaos and founded several powerful dynasties in Northern China. The Tang dynasty absorbed lots of norms of Xianbei and the founder of Tang is half Xianbei too. Xianbei greatly affected how later nomad tribes established their regime and the word Khan was first used by Xianbei highest leader. The Tanguts lead by a descendent of Xianbei noble founded the Western Xia so possibly Xia can be Xianbei in the game and Tangut language can be used(Xianbei language cannot be found).
Dynasties built by the Xianbei generally love Buddhism. The first large scale fervor of Buddhism in China happened during Northern Wei. Temples bloomed over the country to host masters traveling from India and expensive royal expedition were sent to India for monks to bring back Buddhist scriptures(Song Yun). Their wonder can be the famous Yungang Grottoes.
Mulan is a Xianbei.
Civ bonus
Foragers and hunters don’t need mill (like Khmer farmers);
Hand cart and Wheelbarrow can be researched instantly;
When knight line units die spawns militia line units(No Paladin);
Can upgrade monk to imperial monk(+20 hp +2 range);
Team bonus
First monastery free; Monastery available in feudal.
Unique units
Liuzhen Cavalry: Historically noble knights guarding the six frontier towns. These elites also held great political power.
Generate hp like Berserks. High pierce armor and has bonus attack against buildings.
Unique techs
Sinicization: Liuzhen Cavalry generate hp faster and university techs cost 50% less;
Luoyang: Get 100 gold for each monastery existed and relic generate 100% more gold.
(Luoyang was the center of Buddhism in East Asia during Xianbei rule, full of luxurious temples.)
Tech tree focus on cavalry and monastery techs, adjust accordingly for balance.
Hopefully someone in aoe2 team can see this. Personally I think Xianbei fits better in the game than Khitans or Jurchens. Xianbei set an example for later East Asia nomadic people regime, Khitans and Jurchens basically went similar route as Xianbei.