Here's an idea I had just now that really amused me.
I was grabbing a few OSR/system neutral books to read for later, most of them Setting books, and couldn't help but notice that some of the authors of different books I own have had online arguments in the past where it would be safe to say they dislike each other strongly.
For the sake of good taste, I shall not divulge the titles of the books or the names of the authors. FOR THE SAKE OF BAD TASTE HOWEVER, I had a really fun idea!
If like me you have different Setting books from authors who don't like each other, grab their respective lands, put them in your Campaign World and have them go at war with each other! It actually seems to me like it would make for a great campaign.
First of all, wars always make a campaign more interesting, thrilling and dangerous. One only needs to look at movies like Yojimbo or A Fistful of Dollars to see how even a small-scale conflict between two groups opens up many possibilities for adventurers looking for coin, a full-on war even more so. That said, when that happens both sides are normally created by the same author and so even if there is a strong attempt to make both sides different, there's a certain artistic similarity that cannot be shaken off between them. In other words, the two sides share the same DNA and inclinations of the author so to speak, either because both sides are just too similar or because in an effort to not have that happen, the author made sure to avoid any similarities between them. I'm not saying there are no exceptions mind you, just stating the creative pitfalls of such a project.
Anyway, you don't really run into that problem if each side was created by a completely different person with completely different goals and inclinations in their writing. These days these Setting books are not full worlds as much as a land or part of the world to be inserted into already existing campaigns so the work to have them both co-exist in your world should be minimal. All you have to do is extrapolate why they went to war and how each side would react. All those NPCs, locations, magic, etc, how would they react/be used in a war? If these settings were created by authors who disliked each other, it should be fairly easy to find how their worlds, their very creations, also would not see things the same way. In some cases the very real-life conflict that took place can even be projected into the NPCs.
Let them fight!