As I explained here, I think the reason why they cost population is because there are already 2 new shipments of ‘infinite natives’. (14 habsburg allies Age IV+ Army of the Circle Age III).
I could suggest the following:
In addition to the current benefits, it gives you a native embassy.
Units will not cost population until age 4, once you reach age 4 they will cost population. In case the ‘14 habsburg allies’ card is missing they will cost no population, but the number of trainable units should be reduced accordingly.
Once you choose the ally, it gives you a number of units from the chosen royal house. (could have some coin cost).
In addition to the current benefits:
You can train mercenaries and outlaws in batches of 10 at the mercenary camp or tavern.
Re-enable the mercenaries you have sent so far. Infinite mersenary shipments arrive faster.
It could also reduce the mercenary population by -1.
I will not extend myself further because this already has its own theme, many things have been proposed in this regard. (Wallensteins contract needs a rework)