I wasn't too sure what he meant by this. I could take it to mean, "Favorite idea for merging two campaigns into one," or I could take it to mean, "Favorite idea for merging two game systems or settings into one," so I will just take it both ways.
I have a lot of experience with merging games. I have run different groups in the same campaign world, sometimes even the same dungeon, for a number of years. Most recently, I have drawn all the heavily used areas from my recent fantasy campaigns into one world. This was pretty easy to do, I just cut out those sections of the maps and then glued them back together. They key was that by eliminating all of the history that my players never encountered in game, I found I had almost no conflicts that would have to be explained away, even if all of the players involved discovered everything that the other players knew (unlikely). I think they lesson here is that if you only create what you need, and do not get attached to things your players have never seen, it is easy to merge parts of a game world together. The benefit to this is, every part of my game world, that I know anything about, has been developed through play.
As far as merging two systems or settings together, I have to go with the upcoming Savage Worlds version of Rifts. I am really eager to see this.
And finally we are at Z for Zombies
45 minutes ago