Add a second upgraded version for units
If adding a lot of factions after the four we currently have isn't something that is foreseen, I would suggest adding a seconded upgraded version for each unit, in a manner similar to what Heroes of Might & Magic V Tribes of the East did : not only this would allow the introduction of new assets without having to work on them from the ground up, but this would also give more ways to express a faction's identity (and Songs of Conquest already likes to show the bivalent nature of each faction, most notably through the race coexistence aspect) and give players ways to mix and match different combinations of units, multiplying the potential replayability of each faction.
That said, all of this depends on what level of complexity the studio would like to bring to each faction. Plus, it would require a rework of the UI for the unit window to allow both forms to coexist. I would perfectly understand if that kind of idea were secondary at best.
In any case, thanks for you work !