I do appreciate the way rules for the new game factions are going along with the lore and try to both showcase a culture strong and weak points while encouraging some specific playstyle.
Thinking about some threads featuring complaints about upkeep, I'm started thinking something could be done to fit even more the different lore. The game needs some diminishing returns when a faction grows, since income and warfare allow for some serious snowballing. This is why we have a upkeep increase with the number of lords, this penalty becomes quite steeps at higher difficulty to the dismay of some players. Here's my suggestion
Have both a army number upkeep, but lower and introduce a lost taxation system - based on number of provinces. Make the value for the two different for each culture.
Both penalties represent different things : the logistic required for handling new lords and their suites, and the issues that arise from governing a large empire. The later was present in older total war game and is a feature of many strategic games. It could be distance and climate dependent.
The thing would be that destruction aligned faction may have more "loss of taxation" whereas the order aligned would have most burden from the number or armies. This would encourage different playstyles and further increase the feeling of different social order.
Factions of order are able to flourish despite warfare and have to bear the burden of managing armies. This is the present situation. Factions of destructions are more profeteering from war, feature more individualistic lords that see conquest and raiding as a path to glory and power. But their flawed society is their own demise and they can't profit as well from the territory they control. They are able to field more armies in general but can't extract as much wealth from their controlled provinces nor keep public order as well without the use of force.
This is already suggested in the design of the skaven corruption/slavery vs the geomantic web. I feel the diminishing returns penalties could be modified to go along this overaching theme. An added bonus would be to let people mod and tweak both upkeep and taxation penalties individually for each faction.
Make upkeep increase less strong in general, especially for "destruction" factions
Add a taxation malus for large empires, especially for destruction too.
Alternatively : keep upkeep increase stiff but allow some buildings in high tier settlement to diminish upkeep.