I think more solid rules about when each of the coalition have to peace have to be made. Right now it is possible to continue fighting at -3 as for example Austria even if France manages to capture 90% of your territory. Perhaps something like this would be a requirement?
If France has 99 WS on country X, and country X's capital is captured, and all to France human enemy held provinces bordering X's capital are captured by France, then country X has to accept whatever peace France asks.
How it would work in reality: As an example Austria would be force peaced if Vienna was captured and all provinces bordering it, and France had 99 WS. A clear cut case. However, to peace Prussia the rule about "to France human enemy held province" implies France would not only have to capture "only" Magdeburg, Berlin, Küstrin, Hinterpommern and Silesia, but also Erz as it borders Berlin and is enemy towards it.
Additionally I suggest that if anyone in the coalition is force peaced by Francde, it shouldn't be possible to break the truce for that country. So for example, Austria cannot rebuild two years and then DOW France again three years before peace goes out.
A further dicussion on this issue and my suggestions is welcome.