I would like to see three kinds of wars that can be declared: "skirmish wars," "subjugation wars," and "total wars."
In a skirmish war you're battling for some small thing, like a province or two that you have claims/cores on, or a lesser casus belli like a trade war or dishonored call. With these, the war scores change much more quickly, and the AI doesn't have the stubborn "length of war" modifier (or at least a much smaller one that evaporates much more quickly). Certain concessions are barred from being selected in a skirmish war; for instance, you can't take provinces on your target's home continent that aren't included in your casus belli (colonies are fine though), and you can't force vassalize (unless you're able to annex with a skirmish, like having a claim on a OPM). Skirmish wars can be declared without a casus belli, but of course your ability to take stuff is limited (due to the aforementioned restrictions on concessions); you'd mostly do this for off-continent provinces or to pull in their allies who have provinces you do have claims / cores on. Truces from skirmish wars would be a lot shorter; 3-5 years typically.
A subjugation war would be fought between a powerful nation and a less powerful nation. You can only declare these in certain circumstances; for instance, if you have a casus belli to vassalize, or if you have claims / cores on their entire country and you're a certain size larger (say, 5x or greater), or you're a modernized nation invading a tribe, that sort of thing. War score would work as with current wars. Truce timers are mostly irrelevant since the typical result is the small nation gets absorbed, but 5-10 years makes sense for cases where the small nation survives.
Total wars, on the other hand, permit full annexation / vassalization, and you can also take lots more provinces than you can in the game today. This is Ottomans eating Mamluks sort of war. You can only declare a total war if you have an appropriate casus belli, and a nation can only declare one total war every 20* years (but can be declared on in such a manner without limit). War scores and AI peace behavior would work the same as with wars in the current version. War score costs of provinces taken would be something like 1/5 what they are now, and provinces conquered through a total war get a substantial (temporary) reduction to over-extension. This reduction would fade a bit annually, so a conquerer would have to devote constant resources to coring everything possible in order to prevent over-extension from shooting over 100% (but it should be possible to do). Alternatively, Create Vassal could be made available much earlier in the game, and you just split your newly-conquered area into a few vassals until you're able to annex.
Events, Policies, Ideas, and even government types would provide the ability to declare total wars more frequently (events popping to let you reduce the remaining timer in exchange for stability, policies and ideas that speed up the timer, revolutionary governments having no time limit at all, etc).
This would also largely fix the whole problem of the "peacetime game." You don't have a peacetime game; you have total wars, and then you have lesser wars in between them.
* Probably this should actually vary based on the size of your opponent.
