If any modders are around:
So far, the outpost ally's units we recruit don't scale with late game because there is no red tech tree for them.
A solution would be to allow the player to recruit one "foreign" hero/lord at level 3 outpost with the appropriate red branch so these units can be properly boosted.
That would allow fun builds like an Empire rocket batteries army for Kislev or a stronger frontline army for Bretonnia, etc
In any cases, that would not affect the faction balance since outpost units are limited in number so that would only affect an army or maybe two. Heroe/Lord may actually increase this limit by a bit. But nothing too strong to keep the faction balance.
This is just an idea, if any modder want to take, feel free