Spiderman in the Avengers "world"?
I don't usually follow Marvel properties. But, I was curious. I know Fox and Marvel aren't on the best terms and neither would allow each other's properties in the others films. But, their does not seem to be that animosity between Sony and Marvel. So what are the chances they would allow references to each other in the films? Be it a mention of the Daily Bugle in the Avengers or mentioning the destruction of NYC in Spiderman. It shouldn't affect the movie...Actually, it would be a nice fan service. Maybe, they can even come to a bigger agreement to allow characters to interact in the films...Only would increase ticket sales.
Finally, Spiderman and Avengers worlds seem the most compatible to interchange. The mutant world of X-Men would be too different to what is being shown in either Spidey or Avengers.