No reforms at all?

[Watchman]

Generally it just plain works out better - and with fewer problems - to simply go with the realistic enough assumption that most factions taking over lands outside their "core areas" will just recruit the natives to fight for them according to their "national" local traditions. Which is how it worked IRL for the most part - before the active consolidation of their holdings into a formal empire and the slow romanisation process, the Romans too tended to rely quite a bit on client-kings and local warriors in the more far-flung reaches of their realm. Similarly, aside from the actual military colonies (which in a slightly different form had been around already under the Achaemenids) and Hellenised "core" regions the Successors drew quite heavily on their Iranian, Egyptian, Indian and whatever subjects for soldiery, just to give a few examples.