Well, as far as the future of mass effect, I could see it being a sequel perhaps a thousand or so years in the future.
I mean, a poster said something about warhammer being technophobic--never played warhammer, meself. If you pick destroy--you know the galaxy is going to be extremely technophobic for quite some time after the Reapers. There's no way around that.
Even in Control--Shep is off somewhere, and has become some myth. Regardless, the ones left standing are going to still be traumatized by the Reaper invasion. Which to me means they, too, could become very technophobic after all is said and done.
With Synth...well, I could imagine them becoming technophobic IF something went wrong with the synthesis. Perhaps there were some unforeseen side effects? Maybe it won't "take" well. So, if that broke down, again, we could reasonably be at a unifying point of a galaxy that has become extremely technophobic. Depending on what could be imagined as going wrong with the process of Synthesis.
I could imagine a future mass effect galaxy that is very technophobic. The whole thing was extremely traumatic for the races. I could imagine that trying to invent, or being caught with any tech could warrant your arrest and maybe execution, because the fear of what might happen, and the horrors of what did happen.
It could turn to an extreme, zealot type of environment. Black market tech that uses mostly gum and duct tape. Lol! I could see a cult of Shepard that worships the myth, as well as those that demonize Shep--depending on if there were any nasty unforeseen side effects of Synth. Or, frankly any of the other choices. Existence would be extremely difficult for a long time. Shep's actions might not be as fondly remembered as we think they should be. Shep might eventually be blamed for using this crucible tech blindly. Not at first ofc. Everybody's just happy the Reapers are gone, but after awhile, a generation or two, when reality sets in of what they're left to deal with, some resentment might stir.
Populations may become isolated from each other. Perhaps even races that we take for granted as knowing that they exist, might be relegated to the realm of myth as well simply because of their isolation. And as other posters pointed out, this isolation over a good amount of time (tho 10,000 years seems too much to me) would lead to changing cultures and dialects, maybe even physical appearances. Depending on how long it's been and what environments races may have to live in through the aftermath.
I know the EC painted everything as just fine, but it was just fine eventually. The EC had a narrative that spanned alot of time. So, we may be alright eventually, and get back to and transcend what we were before--but that will take time.The relays in the EC may not have been destroyed, but they were still badly damaged.
Resources and populations are gonna be scarce for quite some time. The galaxy was completely trashed. The infrastructure of the mass effect civilization, as we knew it, is gone. Anything goes now. The Asari themselves may be demonized for holding out on the tech--if news of that got out. They might end up being reviled instead of revered.
All I'm saying is that it would take very little to find a common starting point for a sequel because regardless of our choices, the meu will never be the same. Not after what they went through.
I, however, am not passionately tied to any of the ending choices. I pick destroy, but I really wouldn't care if they went Synth or Control. I just don't feel that strongly about any of them, good or bad. I understand there are others that do. That's cool. I'm just speaking for myself here.
I do hope they give us a clue as to their intended narrative direction very soon. I'm about tired of floundering in the dark here.