After many requests and a lot more issues than I expected, I have a test version of the Alexander Submod available that has multiple factions playable. Epirus and Sparta are now available alongside Macedon, so friends can take on Persia together!
Head over to the Alexander TWC thread to check it out. Currently, this version is only available in a standalone download. Eventually, it will also be on Steam.
We also have a new update coming soon for all versions of the Alexander Campaign that will have new captain units featuring historical characters as well as various other fixes and changes. Stay tuned!