Well, starting settlements with less buildings would require me to redo the startpos (which I do plan to do eventually to add some more substantial changes, but not soon). Maybe something I could do, but there's also an argument for leaving the Romans and Sassanids with fairly well-developed settlements to begin with. And I have already reduced growth quite a bit from vanilla, but I could reduce some more of those effects, sure.
Your new raiding stance effect sounds cool! As far as building upkeep, I'd prefer to keep it a fixed number rather than a percentage, just so it's easier to see where your money is going, and it makes a bit more sense. Of course a percentage would be much easier to scale. The reason we didn't have fixed numbers in DeI is because you can't really do that in Rome 2, so we had to find a way around it and went with the provincial income penalties and empire maintenance.