I've been a sucker for the classic crossover. They're now becoming commonplace, (due to the comics taking place in the same universe, repeating it in the movies) but some like the SCP Foundation is something that could take place in any universe.
A few ideas popped into my head when I was thinking:
-SCP Foundation/David Wong's Stories For those who haven't read "John Dies at the End" and "This Book is Full of Spiders", most of the weird takes place in a town called "Undisclosed", and somehow thinking back, I thought about this when I re-read "John Dies at the End". The crossover could be something of a mix between document and David's POV of the Foundation.
-Zootopia/SCP Foundation Now, I personally haven't seen Zootopia… yet. But until then, someone could write something that either has Nick and Judy end up in the world of the SCP Foundation, or have them chosen to join the Foundation in their universe (if going with the theory that long after mankind left, Mammals turned anthro and took over). The possibilities on this is limitless.
In addition, what's your personal opinion on Project:Crossover and what's your favorite story from it?