I feel like we were able to do this with CWA back when it was historical.  We got Ele archers to shoot 3 arrows, one for each archer... but doesnt seem to work anymore.  We had to turn them into "building" AI so they were unable to capture, and then it was when there was random fire and not focus fire.

That is probably beyond my time/abilities but I know it has been discussed. I think it would be a great opportunity to enrich gameplay (multi-projectile siege, unique units, ships perhaps).

EDIT: Fixed. It was these lines:   range/ and stable/ were missing after I moved those techs to these respective folders. 

Parthian tactics is un-researchable even after researching Archery Tradition and Advanced Cavalry for Xiongnu and Scythians.

Choose the 'Removed' option in the player placement dropdown.     Maybe not Illyrians, but we have several more in mind. Some of them are quite mature already in mods and can be ported over to base game in the future. For variety, Scythians and Xiongnu seem like logical candidates.