Having turned my attention back to Cyrus the Great, I recent;y came across an online translation of Ctesias's Persica. This describes (briefly) an expedition by Cyrus against the Derbices, a nation on the Syr Darya. These ambush him with a force including elephants sent by their Indian allies. Aha!
Now, these elephants probably looked more like your average, ancient Indian army elephants than they do the Hinchliffe Sassinid elephant kits that I picked up cheap last year (or was it the year before?) I picked them up, partly because I got a verry good deal and partly because I used to have a pair, a long time ago and liked them even if they weren't as impressive as my massive Garrison Carthaginian beast. The elephants were
one of several units that for reasons I have now forgotten, don't appear to have graduated from college with me.
OK so that's not justification for an elephant IN my Mede/Persian army but it is justification to paint up some up for an enemy and nicely gives me a focal point for another enemy, One that could take part in a (rather stretched) 3 way campaign. I had considered the Saka or Massagetae as a 4th army but they just didn't seem distinct enough and didn't seem to offer anything new for a table top game except the possibility of a swirling conflict of horse archers which is not particularly well suited to my new smaller table which I am planning.
Now I'm not exactly sure what the Derbices looked like or what their army was like and I'd just as soon not know too much at this point (unless it reinforces my imaginings!). A mysterious people in a far away fertile land of walled cities on the edge of a desert? What's not to love?
Given their geographic location, something similar to the Saka and Bactrians seems reasonable. I picture a largely peltast like force supported by archers and cavalry including horse archers, and possibly some chariots since they were still in use in India for some time to come. Dress to be similar as well with trousers, tunic and soft cap. Again, the end result will be troops that can be an enemy or be incorporated into an Achaemenid army. Eventually some real Indian troops could be added for additional colour.
The plan now looks something like:
a) A Grek army, on its own or allied to the Lydians or as a source of mercenaries for the Persians
b) A Mede/Persian army including proto-cataphracts ala Cyropaedia.
c) A Lydian army including mercenaries/allies supplied by Babylon/Assyria and Egypt including arabs as well as various Anatolian troops including Phrygians, Lydians, Greeks, Thracians/Bythnians, and other tribes from the fringes. All of these could also serve as subjects and mercenaries in an Achamenid army.
d) The Derbices possibly with gratuitous Massagatae allies as well as Indian ones and including elephants.