Battle of Flodden (1513) comes to mind -- England gets 100% war score Vs. Scotland, after 1 battle.
My whole premise was simply that if... IF a major battle goes disastrously, it should count for a lot more war score than EU gives you. EU is way to stingy with it's war score allocation in this regard, and so when something like this happens in CK2 (and the game rewards you accordingly), it's like a breath of fresh air.
You seem to have no idea of what '100% warscore' means in EUIV terms (as opposed to CKII ones). '100% warscore' in EUIV would have meant that the Scots would have agreed to English annexation of large parts of Scotland, or other extremely harsh terms. This did not happen; and there is every evidence that the Scottish lords would have fought for years more if such a peace deal had been proposed. It ended the invasion by Scotland, but that was it. Effectively, a white peace. That's nothing remotely like 100% warscore. (Further, Flodden wasn't part of a separate Anglo-Scottish war - EUIV would represent the invasion that led up to Flodden as part of a larger war against the French (the League of Cambrai), with whom Scotland were allied. All Flodden secured was the end of active Scottish participation in this larger war.) So you've got an argument for Flodden - one of the most decisive defeats of its era, and an order of magnitide more bloody than, say, Novara - representing, at most, 20% or 25% warscore. No more than that. More than what EUIV represents, no doubt, but nowhere near what you're advocating.
In EU4, morale is exhausted, and as long as you've lasted 10 days, you essentially have space aliens that beam you up, and transport you far away to some safe place well behind the forts. Why don't you tell ME, when that has ever happened in history.
Sure: Malplaquet. Fleurus. Bicocca. Heiligerlee. Lutzen. It was notoriously difficult, throughout almost the entire era, to achieve a really descisive defeat. The way you got one, both in histroy and in game, is by cutting off your enemy's line of retreat. This could be represented better (for example, retreating over the Alps was much harder than it's represented as being), and I'd be in favour of some tweaking on those lines: but the basic principle is thoroughly historical.
Anyway, I think you could make a case for battles having, say, twice the warscore and impact on war enthusiasm they have now. But your argument as you've presented it would mean that you could attack France with the CB of capturing Languedoc and then annex Paris after winning a single battle, which is just patent nonesense.