This is my first time suggesting this, and I'm not sure this is possible. Is this idea possible to make a mod with?
Terminology:
B = Lowest-level Bonus
V = B's value
M, N = Caps. V must be in between M and N, inclusive.
X and Y = Amount increased or decreased (X increases, Y decreases)
Let's say a territory in B is attacked. V is decreased by Y up to a minimum of M. For every turn that no territory in B is attacked, V is increased by X up to a maximum of N.
My idea here is to add some sort of devastation and/or development to the bonuses. You won't really gain much from something you just recently conquered.

Don't think mods in the current framework can change bonus values during a game. I do like the concept thru.

Rather than change the bonus values (which is impossible), the mod could set all bonuses to 0 before the game, and give appropriate gold per turn for whoever own a bonus.

Would probably need a GUI as well, and sound like way too much work 😅

My idea here is to add some sort of devastation and/or development to the bonuses. You won't really gain much from something you just recently conquered.

I'm on it actually, I'm giving additional gold depending on the bonus level (bonus level increases the longer you hold it)

I'm working with the following formula:

min(MM, floor(BL / BIAT) * BM) * BV 

where
MM = Maximum Multiplier (set in the mod configuration, default = 2.0)
BL = Bonus Level (increased when the player holds a bonus for a turn)
BIAT = Bonus Increases After number of Turns (set in the mod configuration, default = 1)
BM = Bonus Multiplier (set in the mod configuration, default = 0.1)
BV = Bonus Value

You're indeed not able to adjust bonus values during the game. I keep track of how long the bonus has been holded and reset this value when the bonus isn't complete anymore. I also keep track of who owns the bonus since a bonus can change from owner in 1 turn (1 territory bonuses in particular).

I'm currently not developing this mod cos I want to finish some other mods first before releasing this one, and it got some issues.
issues:

• atm it is actually possible to lose gold with negative bonuses
  
• there is an option for exponential growth instead of the formula above, but this needs a lot of work
  

there are also some features I want to add. Currently nothing happens when an attack on a completed bonus fails or a territory is lost but taken again in the same turn.

@player25253 if you like I can change it that you get less income (below the bonus value) if you just conquered the bonus

any suggestions? Feedback is always welcome

What is "lowest-level bonus"?

I assume it is the lowest level a bonus can become. in @player25253 description of the mod a bonus should lose levels when there are attacks on territories in the bonus, so B should make sure the level doesn't go below that value