Benevento? Tagliacozzo? Ummm, Hastings?
Benevento wasn't the last battle of Charles conquest of Sicily. Even more, if you're trying to put an example it's Benevento OR Tagliacozzo, because second followed first.
Hastings wasn't the last battle Wilhelm fighted too. English lords coronated Edgar, and there was skirmishes. Non effective (because some lords don't take new king as rightful ruler and because power of Norman army), but to end a war Wilhelm needed to capture archbishop and subjugate other english leaders.
It's not popular history, because it wasn't very bright. But it was happen.
Bouvines. Or rather, the Coalition fielded multiple armies against the French, each of which where defeated in one battle.
Bouvines wasn't the first battle of Anglo-French war. For example Normandy was lost to French at the Bouvines point. Even if we take only campaign of 1214, Bouvines wasn't the first fight.
Yes, there were
armies defeated in one battle; I asked for
wars.
UPD: Ah, I saw your second post. Yup, it can be intresting, but campaign-braker battles (battles we know as campaign-braker battles) is battles with big warscore impact. So if we can't really do it simple and gracefully, campaign mechanic can be a mess, and it will be essentially flavour only.
And when armies still has cohesion they, by engine, represented as continue to fight - sirkmishes, local retreats, returns and so on. After all, we're speaking about fights taking weeks and, sometimes, monthes on territories about thousands of km2.
I didn't tryed yet - is it possible to make retreat by yourself without losing the combat by current system?
Duchy take overs in England should not concern the Holy Roman Empire, or concern Hungary.
Weren't they concerned in real history?