Martin's and Jennifer’s arcs in the future part of the CTM AU
Because I obviously cannot keep either of them static and the same for the entirety of the story of my AU.
So for starters, they both together do start out as exes. In part because Martin has been a stickler to the rules and a total doormat for even the slightest bit of validation before getting his time machine vision, and also in part because Jennifer wasn't quite seeing eye to eye with Martin when it came to assessing risks that come with breaking rules.
In short: they are polar opposites of ways in how you can do risk assessments wrong. Jennifer is reckless to the point of endangering people unintentionally while Martin is also overly careful to that same degree.
Martin already has gone through a small bit of change by the time he does his test run of the Delorean, but it isn't really that much, since he went to ridiculously extreme lengths to not be caught up till then there.
Jennifer in the meantime is not exactly willing to change yet, since that means conforming to something she absolutely hates (and that also hates her). However in 2016 she does get challenged by the punks there out of all people and that becomes a matter of "hey, being a punk isn't just about the rage, it's also about going against the status quote, which is right now police rage. So it's time to do and be kind for one another, you know?"
Martin in the meantime learns the hard way with his interaction with the cb!timeline Hill valley police (Griff Tannen specifically), that toeing the line and being super duper careful about not being noticed or seen, does not guarantee your safety. And for sure not that of your loved ones.
While i had initially the idea of this leading to Jennifer’s incarceration because she had to save his ass, I think it would work better if he just got caught instead (to spare Jennifer) and Jennifer had to figure out some kind of plan to break him out instead.
So... this would lead to Edna Strickland herself doing a little Citizen Plus treatment interrogation session about the whereabouts of that dangerous time machine, so she can get rid of it and destroy it "for the well-being of Hill Valley". Basically giving up and folding over this time is just absolutely not an option to even consider for Martin.
Additionally the attempted Citizen Plus treatment just keeps on not working for Martin because of his undiagnosed ADHD, since his attention and focus just keeps on wandering to places and topics that Edna cannot control. That is really just testing her patience there, by the time Jennifer Leroy Jenkins herself in to save her nerdy ex (character development).
The rescue mission doesn't 100% go to plan of course, Griff Tannen puts himself in their way and gives Jennifer a run for her money until Martin whacks him over the head with a keyboard, knocking him out. (It ain't followed up by a kiss yet, but they're in each other's good grades now.)
After that ordeal they end up finding Citizen Brown, as they were trying to find a temporary hiding place in his room. Brown of course goes at first like "Michael/Harry/Sonny?" in complete confusion upon seeing Martin, before then seeing Jennifer and then going "Oh. It's just you Martin. Anyways, go hide in here." And he points to a hiding place in the room.
Edna barges in with some policemen, asks citizen Brown where they are, he just says "No one came in here, what are you talking about?" Maybe acts up a little bit intentionally senile to get them off Martin and Jennifer’s scent there and it basically works.
When they are alone again, the first thing Citizen Brown says is "Congratulations on you temporal experiment, Martin." Which of course baffles both teenagers there, considering the last time they saw him, he desperately tried to prevent them from doing that time jump. And in response to that, Brown basically has a little Monologue about his life regrets, how he had come to realize that the past 30 years, how sorry he is about all of that, to what he had enabled Edna to turn Hill Valley into and that it hasn't always been that way, etc. In short he is low key asking them both to travel back in time to prevent him from doing these mistakes in the first place and hands them Mr. Fusion. Citizen Brown does by that point suspect that some time travel fuckery had happened in his own past, but he can't quite pinpoint what or how and is on his deathbed anyways, so he just wishes them two kids good luck on getting home and/or correcting his mistakes.
Martin voices some uncertainties, considering so far that boy knows, they've only royally fucked the future up with that experiment. Citizen Brown retords to that with "the only thing you did with that experiment was to succeed in proving ME wrong, which doesn't happen that often."
Anyways, after this talk and installing Mr. Fusion to the flux capacitor as a new energy source Martin and Jennifer jump back to 1986, so they have room for breathing and trying to figure out WHAT mistakes Citizen Brown had been talking about and more importantly WHEN they've happened.