One of the coolest sections of Back To The Future Part 2 involved Marty having to travel back to 1955, shadowing his original trip back to the past from the first film but in such a way that he didn't come into view or change anything that would affect his previous self. Of course, this section almost didn't exist at all - fading away like a polaroid that adjusts itself for some reason.
In the original script for Back To The Future Part 2, Marty and Doc had to go to 1967 instead of 1955. The thinking was that the series had already explored that portion of Marty's parents' lives, so why not try out a new time period for Marty to try to awkwardly fit into and invent music for? in 1967, Marty would find his parents as part of the hippie movement, and again Marty's presence threatens to prevent his own birth (and the time-ruining sports almanac would still appear, but start off in 1960 instead of 1950).
Why was this scrapped? Well, going back to 1955 was a lot more fun, story-wise. And weirdly enough, the plan to travel to 1967 and having to ensure his parents bone is MORE retreading the same ground that going back to the same year as the first film. And lastly, the friction with actor Crispin Glover (who played George McFly) made him (and the character of George) basically unusable for the film. Instead, we got some fun exploration of paradoxes and a Crispin Glover lookalike hanging upside down for about 2 minutes.
Groundhog Day is - for an early 90s romantic-comedy - one of the most existentially horrifying films ever produced. A man is trapped in an inexplicable timeloop for no actual reason, caused by circumstances and powers he can't even begin to fathom, and not even his own repeated death can save him from his Kafka-esque fate.
Of course, eventually the magic of True Love saves him from the timeloop for some (also unexplained) reason...but originally Andie McDowell's love DIDN'T save him from his hellish predicament. In fact, it made it even more terrifying, as Andie McDowell was now trapped in the timeloop as well.
Worse, the original script actually told the audience how long Phil was trapped in this Twilight Zone situation - roughly 10,000 years. TEN THOUSAND YEARS, OF REPEATING THE EXACT SAME DAY, OVER AND OVER, WITH NO HOPE OR MEANS OF EVER ESCAPING. That's pretty messed up, and might help shed light on how Bill Murray became Hollywood's equivalent of a wily trickster god.