Currently, the cost added for more forts does not take into consideration the active player the nation has, and should in my opinion.

I'm proposing to have the scaling also be affected by a nations active resident count. Somewhere in the range of per 60 ACTIVE residents, the scaling is decreaed by a factor of one. It should not go off of total residents, instead only going off of the ACTIVE residents.

For example:
Nation (A) has 32 active players and has 3 forts. They pay the normal 3 forts = +x% + x% = +x% upkeep

However, for Nation (B), it has 117 active players, and has 5 forts. They should only pay the same +x% + x% = +x% upkeep, with the 2 reductions of scaling coming from its active player count (2 reductions, 1 per 60 active).

Along with this change, To prevent actual over expansion, The scaling should go +4%, 8%, +12%. +16%, etc. This, so that if you DO decide to overextend without a real reason to internally (to have more space for your nations residents), You will still wind up paying for it.

TLDR; This entire change would look something like this.

Nation A- 3 forts with 32 active players, They would pay an extra 12% in upkeep for not having sufficient active players to justify the multiple forts.

Nation B- 5 forts with 117 active players. They would also pay an additional 12% in upkeep, with 2 of those "increases" being negated by the 2 reductions for their active nation member count.

It would promote nations to grow sustainably, not kill their growth with, and still punish the single use forts certain groups are known for making.