Even if I don't have Rome 2, The greatness of your mod which name has spread throughout the Total War Forums have made me filled with awe.
Haha, Perhaps I even buy the game just to try your mod!

You made Qin units, but Rome 2 Timeframe of 270BC-30AD means you can also make some Han units too.

I even say you should make a Chinese Faction just for Custom Battle only. No need for Campaign.

I see you have made some units using Assembly Kit Model Exporter, which means you can insert new model, but not animation, am I right?

And so, I suggest to you:

- Composite Archers
Yes, I know you cannot make Crossbowmen for now, so why don't you just make "normal" Composite Bow Archers? At least you can make Chinese Archers in Rome 2, even if not Crossbow. They are also used a lot in Chinese Armies alongside Crossbowmen.

-Ge Halberdier
They can use Sarissa Pikemen animation for Pike Wall Formation combat, and a mix of spear and two handed falx animation for close combat.
But I don't know if Pike uses the same skeleton as Falx or Rhomphaia?

-Chinese Chariot
If you can make new model, I sure you can make new Chinese chariot rigidmodel engine.

-Chinese Cavalry
Your Chinese faction always need some cavalry.

-Siege Crossbow.
Because you cannot make Handheld Infantry Crossbow, why not make Large Siege Crossbows for now? Small using Scorpio animation, Large using Ballista animation. Maybe Repeating Siege Crossbow using Polybolos animation - Lian Nu or Large Zhuge Nu?

-Chinese Traction Trebuchet/Mangonel/Catapult
Using Onager animation.