I’m not sure I’ve ever thought of smashing together two games — settings, sure. I’ve done that tons of times. But merging two games? Aside from lifting the occasional cool mechanic from one game for use in another (I have a system for using FATE-style Aspects with Savage Worlds, for instance, which I’m considering releasing in a published product down the road), or playing a game with rules I like, using the setting from a game which has a less-than-stellar rules set, I can’t really envision doing it.
Actually… as I’ve been typing this, I’ve thought of a merger which would definitely interest me: Take the astoundingly complete (700+ pages) STARBLAZER ADVENTURES, the “rock-and-roll space opera adventure game”, which used the original version of the FATE rules with a license for a UK anthology comic series of the 70s/80s. I worked on that game, and I’m very familiar with it. The license has expired, so all of the comics-specific material would have to be excised, but the space opera mechanics for FATE were really excellent. I would take those, bring them in line with the newer FATE rules (perhaps some elements of FATE ACCELERATED, too), and come up with an original setting that reflects late 70s/early 80s space opera. That would be fun.
Would I consider that my “favorite idea”, though? Again, not sure I have a favorite.
Dave Chapman has some ideas…