I can't remember exactly how old I was when I first saw Back to the Future, or upon which of my many rewatches it was that the fixation for this franchise and its characters finally took hold, but what I do know is that for over a decade now I've had a strong passive interest in the trilogy and all connected media.
Besides maybe the Terminator movies, BTTF was my first real exposure to a film with a strong focus on the idea of time travel. Despite not being the first to expose me to such themes, it's undoubtedly the one to imprint on me the strongest.
The way the movie is written both in respect to time travel and the character writing is almost indescribably good, and it's one of those things that I can put on time and time again without fatiguing at all!*
*I would know --- there was a week when I watched the entire trilogy 4-5 times to show different people due to conflicting schedules not allowing a single watch party.
Though my actual dive into the consumption of fanmade content is very recent, and my creation of fan content is even more recent (only a week or two at the time of writing this!), I've got many ideas to share and MANY headcanons that have drifted around my mind for years upon years at this point. This is the nexus for ALL of those thoughts and ideas!
What if things went a little differently during the end of Back to the Future II? What if, despite Marty retrieving the almanac without taking a door to the face or going on a crazy car chase, time still decided to punish him and Doc for his meddling with a lightning strike to the Delorean as it parked atop the roof of Hill Valley High School? No letter to Marty to be found, and no signal from the out-of-time Doc to signal to him what his next step is?
What if the only thing Marty could think to do in order to get home was to chase down his past-1955 self and the Doc from 1955 actively helping that Marty get home? Could it actually work? Or would his attempt to get home only result in disaster on paradoxical proportions?
The trip to 1885 doesn't go anything like Marty planned.
Lorraine eavesdrops and invites herself to come with him. The train hijacking goes awry in the worst way possible, and somehow he ends up holding his mother's hand as his very existence is unraveled.
Major character death and descriptions of fatal injury within!
Even though Marty manages to get his parents back together, he isn't able to quell the ripple effects of his actions. Of course, time alters in the ways we're already plenty familiar with, but what if somewhere amidst those shifts in the natural flow of time were changes previously unseen altogether
The Emmett Brown of the 1950s, full of knowledge of a time machine he shouldn't be remotely this intimate with for multiple decades, decides to put whatever knowledge he's retained (and the blueprints of the time circuits from 1885 Emmett) to the test with the development of an early time machine. Something he knows almost certainly shouldn't exist, and yet something that the scientist within him can't help but try to create --- he can simply stow it away and remake the DeLorean to original spec later, he tells himself.
The Lorraine Baines of this new time grows curious of the man that Calvin "Marty" Klein had been staying with; having seen the supposedly crazed Emmett Brown up close and finding him as (relatively) normal as anyone else in Hill Valley made her want to know just what it was that had people so wary of him... and perhaps having an untapped curiosity for the scientific that would be frowned upon for a woman in the fifties. Could showing up on his doorstep day after day with the same persistence she directed towards Marty work towards getting her a role as his new assistant in his latest project? Apparently so!
Assuming that it's maybe through Lorraine's time as his assistant that Marty one day comes to know him and takes over the role (with no way to know that's the farthest thing from the truth that could be), Emmett brings Lorraine on an anomalous journey through time and sets in motion many shifts in the natural course of events --- the results of which one could never be expected to predict.