5. Back To The Future (1985)/Bill And Ted (1989)
This would be both excellent
(see what I did there? If so, well done, you can definitely be my friend). Imagine, if you will, that whilst traversing the time stream in their respective time machines (the DeLorean DMC-12 and the phone booth), Marty McFly (Michael J. Fox) and Doc Brown (Christopher Lloyd) inadvertently bump in to Ted 'Theodore' Logan (Keanu Reeves) and Bill S. Preston Esquire (Alex Winter). I could probably stop there as far as justification goes to be honest, because the interactions would be amazing! But I won't. If we thought that the problems Doc and Marty caused in the past were detrimental to the future, just imagine the mess that would be made with Bill and Ted in tow. Imagine Biff's interactions with Bill and Ted. Imagine Doc and Rufus (George Carlin) discussing time travel. Basically, it would be awesome. And you know it.