The Old World of Greyhammer

Despite their differences, the Old World of Warhammer Fantasy (at least as presented in WFRP 1e, which is the extent of my knowledge of it) and the World of Greyhawk have enough similarities, I think, to make a mashup both easy and interesting. Both have Andersonian/Moorcockian Law and Chaos alignments as a feature with Neutral a reasonable position, though it's not generally as militant a stance in the Old World as it can be in Greyhawk. Both have the Tolkienian compliment of races without the Tolkien-inspired epic. Both have an Empire that seems to take some inspiration from the historical Holy Roman Empire. 

The way history would go would be something like this: The technologically and perhaps magically advanced Old Slann open warp gates on Oerth, letting Chaos in and ultimately destroying their civilization. Their degenerate descendants are the scattered tribes of bullywugs. Human civilization arises relatively quickly through the rediscovered learning of this elder race, but the Baklunish and Suel eventually use that knowledge in mutual destruction.

Some centuries later, Oeridian tribes establish the Empire (combining the particulars or the Empire of Warhammer and the Overkingdom of the Flanaess). It persists to the present day, but it is a shadow of its former glory. Both all too human failings and the taint of Chaos have a hand in this decline.

I'm not sure what to do with the Chaos Gods, but I'd be inclined to replace them with Greyhawk deities, though I don't know exactly which ones.

I feel like this is a scene that could happen in both settings

The Old World has fewer countries, and they are more obviously analogous to earthly nations, but I don't think it would be particularly hard to map any particular aspect of the setting one wanted to a Greyhawkian surrogate. The default technological era is more advanced, too, but I don't really think gunpowder would spoil Greyhawk in any way. To honor the war game focus of Warhammer, one could also set the campaign during the Greyhawk Wars.

Neither setting is a stranger to punning humor, so there's that. Warhammer is grimmer than Greyhawk at standard, but I think the tones of both can sort of meet in the middle.