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There are TV shows from before about 2001 with entire episodes devoted to one character trying to find another.
The word “streaming” would be very amusing to 1991 me.
Oh yeah, people going to malls.
Yup.
Doc: Marty, we need this part.
Marty: Ok, let’s get it from Amazon
Doc: South America? No…we should head to the mall. We can get you clothes while we’re at it. Kids don’t wear such drab colors around here.
You are right that perhaps I am overlooking the changes in play now that will seem so fundamental in 10 years. But I don’t think self-driving cars are close enough to be that thing.
If autonomous cars coupled with an electrified transportation network and ubiquitous public transportation made getting anywhere almost free, I could see losing that being a shock akin to losing the internet and social media. But self-driving cars are still decades away. The average car sold today will be on the roads for 20 years or so. The average age of a car in the US is reportedly 11.9 years. So, roughly, for every new car there is a 24 year old car on the road. Kids in ten years will have plenty of experience with non-self driving cars even if some of them have recently gotten used to riding in a few of them by then.
As an aside, I think the average car age statistics in the US actually understates reality. For a long time, the reported average age of automobiles statistic from IIHS assumed any vehicle over 20 years old was no more than 20 years old, which tended to skew the average down. It didn’t matter much when typical new cars didn’t make it anywhere near 20 years and when auto sales were growing rapidly but, in a time when car sales have plateaued or dropped and the many of the remaining cars are well over 20 years old (I have one that is 56), the skew today could be significant. Unfortunately, the underlying data supporting the IIHS’s age estimate is no longer readily available so I can’t see if they changed their methodology but there is no single year giant bump in age that might make a methodology change apparent.
You could do a nice riff on the scene from the original where Doc marvels over the camcorder by having Marty show him the movie he was recording and then have it turn off because it’s out of battery, and of course Marty has no idea how to make a USB charger, so it’s dead for the rest of the movie.
(from the linked thread)
Could also do a nice variation on the original by having Doc be a cynic and not the least bit surprised by this.
I think the biggest challenge to such a movie is making 2021 Marty cool and capable compared to the kids from the past. 1980s Marty is cool compared to the 50’s kids (particularly his parents, but all of them). He’s in a rock band, he’s capable on a skateboard, he’s independent and scrappy.
When we talk about the cultural changes between the 90s and today, things like “plans are fluid” and “we can contact anyone we want at any time, or have information at our fingertips”… those don’t make you cool compared to the past. You get some fun fish out of water moments when the future kid is without their toys and internet, but that’s not enough.
One plausible avenue you could take is that 2021 Marty is an influencer/youtuber, and uses his film/marketing skills to do something cool. There are lots of teenagers today with filmmaking skills that would rival professionals in the 1990s, and the That’s one way in which someone in modern times could inspire 90s kids to all think he’s the greatest.
Chef’s kiss.
Teenage kid from 2021 travels back to 1991 with his iPhone.
“Hey , I’m from the future, and check out this amazing technology-- it lets you talk to anyone anywhere in the world… and make video calls… and instantly look up any information you want!”
“Oh yeah? Wow! Let’s see it do all that stuff.”
“Uhhhh…well, it can’t do that in 1991, but it’s also a camera without film… and a calculator… and a voice recorder… and a music player… and you can film and watch videos…and play games!”
“Um, yeah, we have stuff that can do all that too…but maybe not quite that small. Cool…I guess?”
Marty: Fine, we’ll take an Uber.
Marty: Fine, let’s get a Lyft
Doc: Why do we need a lift? I have my car right here.
That’s a bit too cynical.
That line was in the original movie: “Amazing, a portable recording studio”. Extreme miniaturization is still cool. A person from the 50s might have conceivably been unsurprised by the advances in video recording technology over the course of 30 years, but the movies emphasized even incremental changes in tech.
I don’t think that self driving cars will be universal in 10 years, but they will probably be pretty prevalent. Uber and Lyft will probably have transitioned almost entirely to them, so a teen of the 2030’s would be used to hitting a button to their phone*, and having an autonomous car show up to take them where they want to go.
*assuming that they even have to do that, rather than just think it.
There would have to be some misunderstood slang. “Woah, that’s heavy, Doc!” could be updated.
Doc: It turned off. What happened?
Marty: It’s out of battery!
Doc: [Holding assortment of AA, 9V, watch batteries] What kind of batteries does it take?
Of course, the reaction to Biden being president in 2021 would be, “Good for him, he finally won.”
He could invent the modern meme.
I canhaz chezeburger isn’t cliche yet. Nor is keyboard cat.
My point was just that 2021 tech wouldn’t be all that science fictiony in 1991, especially devoid of the telecommunications infrastructure to show what it could really do. By 1991, mobile phones had begun to evolve and shrink from bricks down to candy bar size, though they weren’t yet the technological Swiss Army knives that smart phones would become.
This is an interesting take and I like your suggested plot point. Kids are much more media savvy than kids of the recent past and could use that to their advantage.
The thing that makes todays’ kids cooler to me is that they understand that there are people very different from them in the world and that they too can be different if they choose.
There’s some areas that don’t contrast. You go back to 1991, kids are still listening to vinyl. 
Wait 10 years and the contrast might be severe. “You mean you had elections?? Like a communist country? Trumpmerica wasn’t a fascist dictatorship? And who are all these people with dark skin? And your women aren’t covered? Blaspheme!”
Actually, that might make a nice satire, like Idiocracy.
I don’t know - I was there, and I would have been very impressed. “Wow, the future is everything we imagined!”
Marty: Doc, I said an Apple store. This is a greengrocers.
Yeah, but so what? The idea of a portable video camera wasn’t exactly earth shattering even in the 50s. On-site TV reporters were a thing. It was hardly science-fictiony itself but still emphasized as if it were something amazing. The ability to play back immediately on a TV might have been neat but not something that was barely conceivable. Just smaller and faster.
BttF had ‘fish out of water’ elements. That’s it. You can definitely still do that now, even with an iPhone. That same scene in the original could be duplicated almost directly. The technological advancements would be about as shocking given the technology of the era. It’s played up, but of course it would be for a movie.
You could do something similar with 4k resolution in the phone vs a 90s TV too. Comparing back to the b&w to color transition.