what would any potential plot be?
My attempt:
It could start 2015 (only set in the flying car version of 2015 in BTTF2) Marty is sat on a bench next to the 3D Jaws19 video wall, waiting for a (CGI) 1985 Marty from BTTF2 to walk past, once he sees him he strolls the other way to where he knows the Deloren is parked, gives Biff a dig “not this time Biff” and heads back to the old west to see Doc ‘one more time’.
In his hand is a scan of an old newspaper with Doc’s Obituary, Marty sets the time circuit to a few days before, hits 88mph and disappears.
Back in 1915 (30 years on from the time of BTTF3) Marty goes to Doc who tells him off for messing with the future / past again, and warns him that you cannot really fix time as they thought in earlier films – when they changed things previously they didn’t fix anything, they just changed it to nearer the way things should have been but it was still a new timeline and because of the butterfly effect people they’ve never met have suffered very badly because of their actions or never existed at all.
However Doc is pleased to see him, Marty offers him a 21st Century remedy that could prolong his life by decades, but Doc tells him Clara passed a few months back and how the Old West is dead and he’s not looking forward to the future any more – The Great War is just starting and all the horrors that will come with that, but he’s enjoyed life since the events of BTTF3 and there’s a bit of a Forrest Gump esq montage of him between 1885 and 1915 witnessing some big events, but unable to do anything to change them through fear of altering the timeline, he does however admit to giving Einstein a few pointes in 1905 with a wink and mutters “Annus Mirabilis” under this breath.
Doc dies, his final words warn Marty that the only way to really fix time would be to have never meddled with time at all Marty buries him next to Clara and returns to the cave where his stashed the DeLorean, the same one his used back in 1885 and finds and old arrow under the dust he smiles and kicks it away, this time the car starts and he’s heading across the plains heading towards 88 MPH .
Back in 2015 and things aren’t how he left them, because Biff didn’t go back to 1955 with the Alamac, Marty didn’t go back to fix that loop which changed the time line again – every action has consequences, Marty learns that you can’t simply just go back, change a few things and ‘fix’ the timelines – you can only create new ones and begins to accept the current version of 2015 (which is fairly shit) is the ‘correct’ one even if it’s not, after a while though he realises the true horror of the current 2015 and decides the risk is worth taking, heads back to 1985 .
In the final scene we see current day Marty sneaking about the car park of the Twin Pines (which denotes it’s the origal timeline) mall where Doc and Marty filmed the first time travel, we see them in the background as they were in the first film – after Einstein returns after a minute of time travel and whilst Doc and Marty are distracted 2015 Marty breaks something in the back of the DeLorean – the Terrorists arrive, Doc is killed, Marty narrowly escapes in the DeLorean, but this time it doesn’t time trave, he just heads home to find the FBI at his door – they confiscate the DeLorean, remove the plutonium and crush it as a unlicensed dangerous device. 2015 Marty fades out of existence, this time, the original time line is resorted and 1985 Marty looks to the future unknowing what it will bring.