I was thinking about something similar a few days ago, while watching the War & Peace BBC series.
I got some brief ideas for an ambition system:
A country above a certain development level (say 300) must choose conquest ambitions (They get 200% AE from any other CB). They can mark it as provinces within a nation, or an entire nation itself and there is no limit to how many development worth of provinces are marked as ambition. Special claims are generated automatically on these provinces over a period of time.
An ambition value (AV) is calculated from the development of marked provinces - 1 development counts as:
1 AV - if the province owner is no more than 2 mil techs behind
0.5 AV - if the province owner is between 2-4 mil techs behind
0.1 AV - if the province owner is more than 4 mil techs behind
Ratio of AV to my own development affects many important factors (assume I have 300 development).
If I have Napoleonic Ambitions (300 development takes on 1500 AV i.e. 1500 development worth of similar mil tech nation), the ratio is 5.
If I have ordinary ambitions (300 development takes on 300 AV, the ratio is 1).
If I have minor ambitions, (300 development takes on 30 AV, the ratio is 0.1).
The ratio affects the following:
Time required for generating the special claims (i.e. time required before I can actually declare this war after choosing ambition):
1 year if ratio >= 5
5 years if ratio=1
10 years if ratio <=0.1
Province warscore cost:
10% if ratio >= 5
100% if ratio=1
200% if ratio <= 0.1
Province coring cost:
10% if ratio >= 5
90% if ratio=1 (10% discount from claim is added here - similar to current wars from claimed provinces)
200% if ratio <=0.1
Province contribution to overextension:
10% if ratio >= 5
100% if ratio=1
200% if ratio <= 0.1
AE per development:
10% if ratio >= 5
100% if ratio = 1
200% if ratio <= 0.1
These values are boundary values and the actual values scale according to the actual ratio.
After the special claims get generated, you get a special CB on the strongest marked nation (with all the marked countries are co-belligerents), and all the countries that own the marked provinces automatically join a special coalition against you, and the strongest nation in the coalition gets a CB against you.
Additionally, if you call in allies (say you're France and you choose to call in Ottomans and Russia, then the AV is recalculated based on your + ally development, instead of your own development).
You can make separate peace with all the nations, but the warscore demands from each nation cannot be more than:
1. overall warscore
or
2. warscore against the peacing nation
whichever of the two is lesser.
So if I am France and fighting a marked Austria and PLC, and have 100% occupied Austria (100% warscore) but only 27% warscore in overall war, then I cannot demand more than 27% in a separate peace with Austria. Alternately, if I have 27% warscore overall but only 10% against PLC, I can demand only 10% from PLC not 27%.
The benefit of such a system would be that it would allow nations to have higher risks/higher rewards options, while punishing strong nations for picking off minors one by one to build a power base.
Taking on a similar strength nation (ratio = 1) would be the same as a conquest war currently.
Additionally, every nation can see ambitions of my country, so my rivals and hostile nations will have additional incentive to want to ally my targets.