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. 

Extended to wherever it makes sense would save headaches. Regarding tribal confederations, it would be nice if the choice tree corresponds to probable historical alliances, usually driven by geographical proximity or customs, resulting in a less heterogeneous and more specialised confederation.