Honestly, a simple mechanic that stops wars from being able to mechanically takeover an entire realm (unless they started small) makes sense to me.
The cultural idea also makes sense. Make it that much more difficult to takeover regions next to a capital. And make it even harder to hold onto regions far away from capitals.
If I can't take them, I will drive them rogue. It doesn't matter if I can have them, the important thing is that I deny them to you.
The idea of limited wars in a good one. But t is very hard to come up with some way to enforce or incentivize this. Tim's "grand cycle of war and peace" idea is a possible one that could have some good effects.
