I do play VickyII a bit, and they changed something recently in the economy -- suddenly maintaining a big army ceased to be a nobrainer and the world became a calmer place, and when I played as Spain practically every major campain resulted in debts so I couldn't warmonger around as I used before even as a smaller nation. So maybe there is a point there that (1) waging war should be more costly. I don't mean no fun, it's not a yes-no issue, you can scale it. In EUIII AFAIR you rarely go into financial troubles because of protracted war. There is that silly war exhaustion, which is so abstract hardly anybody know what it really stands for, but no other consequences of protracted conflicts.
(2) On the other hand the logistics doesn't even pretend to mimic the reality: for example the troops get automatically restored to their full strength even in enemy territory. If they didn't and were really only shrinking (enemy territory, so diseases; mountains imposing a heavy supply limit), yet still drain drain your money, you might consider conquering some Albania not worth the price. Especially if your taxes are annoying your own population and you'll need years to pay your debts anyway.
(3) The military system as it is now, means that the one who can muster more troops is the winner, and the winner is usually known even before the war starts. I don't remember a campain that failed because of bad leadership or difficult terrain and even strategic decisions are usually not that important. I find it wrong. History knows many examples of battles where beeing twice the numbers of the enemy didn't guarantee a victory. Terrain, leadership, technology and often luck turned out decisive. Some kind of determination modifier wouldn't also be a bad idea if you wage a defensive war and defend your cores in your territory with own population or even troops recruited in that province. Maybe there should be some measure of righteousness of the war and it should influence the battle, it would measure how much the troops are convinced to the cause. Are they just peasants forced by their lord to invade their neighbour, or do they defend their own land against dirty infidels, heretics or other barbarians.
(Anyway, maybe this way wars within HRE would be also largely prevented without the clumsy mechanic of "emperor demanding return of territory" and the like -- nobody would be determined to fight in an unjust war of conquest.)