As a kid I loved both The Flintstones and The Jetsons. So a cross-over movie was actually a really cool idea. While the concept should never be over-done, I think most fans love it when characters from other worlds collide.
I don't remember everything since it's been ages since I've seen it, but I remember most of the plot had Fred, Wilma, Betty, and Barney in the future after the Jetson's time machine sends them instead by accident.
While the Jetsons are stranded in a land without much technology. But George managed to do pretty well anyway. Mr. Slate sure liked him way more than Fred.
Is there was any beloved cartoon movie from my childhood, this one would be close to the top. I feel nostalgic just thinking about it.