I played a near PERFECT example of what could become a multi-genre game recently, If you guys are wondering, it is The last stand: Union City.
If you haven't already, go check that game out... Its as close as realistic as one can get to my multi-genre concept.
Technically it's a combination of both RPG and Shooter. Im not sure if it is also part Defense game, because at no point are you fending zombies off from a specific location.
IF some defense game aspects were thrown into TLS: Union city here and there, then it would be a PERFECT example of what I mean by a multi-genre game...
Example: At a few points in time you would have to guard a gate or wall to prevent zombies from climbing over and infecting anyone behind it.
When I first came up with this i though that it would be possible to merge all of the game types into one... Now i realize that was a rather rediculous thought.
My definition of a Multi-Genre game? A game that has the aspects of three or more types of games, such as RPG, Shooting, and Defense, all at once.
Hope this clarifies some questions...
-DECA-FADE