In Robert Altman's "The Player" there is a scene where people pitch movie ideas to Tim Robbins. To help him to understand the concept, they summarise it by saying that it is a combination of other movies ("Out of Africa meets Pretty Woman" is one example). You can see the scene here.
I love this idea. It's so cheesy.
But not totally unrealistic.
The idea for "AVP: Alien vs. Predator" must have come from somewhere! (I can hear the pitch now: "What if we had a movie that combined both...")
And famously Gene Roddenberry pitched "Star Trek" to studio execs as "Wagon Train to the Stars"!
It is interesting that the memory of this fact seems to have outlived the memory of that TV show.
Here is an image I put together which shows how "Wagon Train to the Stars" might have looked:
