realistically, everypony from the show is dead other than Celestia and definitely Discord because of what we know about it, because of the show many times, Celestia has prepared things and they always went right eventually, and even when everything else happened and she lost her magic and other things in many different ways, she still knew internally that everything was going to be alright, the biggest proof of this is the scene in season 4 where Celestia uses some weird psychic magic in order to be able to see exactly what's happening with Lord tirek in her dreams, which means that she would know for a fact that she would never be able to defend the country, but she wouldn't be able to stop anypony from fighting about it, or whatever the problem was, so she would sit back and defend the home base hoping for a miracle with whatever problem that made everything falls apart and then she would evacuate, and when the magic disappeared, she would go back to the tree of harmony. (that is only probably going to be referenced in the so as a easter egg, and no, what you saw in the movie is not that, and it was a magical Crystal Tree instead of a living tree.)and then she would be able to use the funds up there to give her enough magic to live there where the tree is and survive long enough in order to come back eventually, probably disguising herself as a unicorn after the movie.
the thing is, even if that would make a lot of sense, that is definitely not going to be in the show in any way at all, even in a small easter egg, and anything that looks like it is not supposed to be it either, the main reason is from how the show is probably going to go from the movie, the most we will probably overall get is a episode about the past and then another episode about that too but another thing at the same time, and it being mentioned occasionally sometimes, and pretty much everything that will be showing will be very genuine and not facts at all, unless the show ends up going on long enough to have a time travel episode, and even then, that will not explain anything that people actually want to know.
the main reason I think this is, is because of the opening of the movie, if they actually wanted to explain anything in a detailed way then they would turn the entire thing into a dream from filly sunny and it would so the main six in all their glory in 2D, but then it would cut the third person and she would see some of the event of what happened before everything fell apart, eventually one of the ponies would start speaking in her father's voice to Twilight or something and then her father would wake her up and then they would talk about the strange dream and continue on from there in a slightly different way, but the rest of it would be the same with maybe a few hints everywhere too, especially in the after credits scene, it would definitely be more than it is now.
there are a few ways they could do it but it would be more or less similar to that, the fact we didn't get that at all basically means that they do not care at all about continuing the story of G4 in any way at all, from how they did it, it makes a lot more sense that they're just milking it in order to get more money from the next product, which means that generation 5 probably isn't going to last that long, and will guaranteed have a bunch of criticism, from the many episodes of the seasons to come.
however, they will have to explain Discord, even in just a Easter egg or a lot of Easter eggs, it might even be mentioned in a episode, with the reason probably being related too: (everything became boring and he lost everything so he left Equestria forever) the reason is because the retcon is so insanely stupid and big that they will have to address it in a way, mainly to keep the series going a bit longer, and hopefully some of it because some writers really want to put something like that in because it's a giant retcon, hopefully...
"where is Spike?"
he is either in the dragonlands, still alive or dead and he probably will not be mentioned at all, at least what happened after the show won't be mentioned at all.
"what happened between the years?"
it is pretty much guaranteed to be a event, and it has entirely nothing to do with any villain that has ever existed earlier either, the reason is because of every villains desire and what they wanted for pony society, even cozy glow would want everypony together to worship her instead of them separating and she would have a royal line that would do everything that she did, so it was definitely something completely different from anything else the ponies have faced before, and it was probably less of an enemy and more of a event, even if there was a enemy, or lots of enemies, then they would be creating a event.
after the event happened and all of the important ponies, or just the alicorns, other than the one that I already talked about, were gone, then the fallout of the event would cause the pony to separate back into what they were before Equestria was founded, eventually they would fear each other because of thinking that one kind of pony encouraged or help to make the event happen, eventually, the three tribes would be completely separated, loved ones included, and then all of the ponies magic would disappear soon after.
and of course the ponies would blame each other on the lack of magic without knowing exactly that everypony else didn't have magic either.
also, I really don't think that Twilight made the crystals, because if she did then what the ponies saw on the stained glass window in the movie would be very obvious that it was friendship, to the point where the entire plot would be broken, no, I think a talented unicorn from the sparkle family made the crystals in a emergency without knowing exactly how much friendship magic works, unlike Twilight Sparkle did, after everything she did before the event, Twilight could have asked her to do it, but that was the last thing she wanted to do, and she would never sacrifice herself to make them because that doesn't make any sense, The Elements of Harmony never needed sacrifice and never really needed themselves in order to be activated, and sacrifice seems to be a bit more like the complete opposite of what the Elements of Harmony or crystals that got activated from Friendship would ever be.
I hope you like my theories too!