The trailer did not explain everyone, there will probably be many hints for the fans as with the movie, which we can then speculate about.
What is prevalent here is the fact that the magic in G5 is likely to be unstable and erratic in contrast to what is seen in G4, and the Unity Crystals are either meant to stabilize the magic or to maintain the Ponykind as a united people. The earth ponies did not get magic, which is difficult to comprehend, and their dissatisfaction almost overturned everything for the ponies who were dangerously close to disaster. Also, the hints that can be seen in the trailer are reasonably clear that the ponies may have forgotten all about the past and that the magic is sometimes very dangerous.
We know that Discord took the Unity Crystals with him to get rid of the magic, which suggests that something is wrong with the pony magic as he, even in his grief over his friends, is aware that he will never return to Equestria if the magic disappears . When Sunny discovered the crystals on the glass door, they were not united together, and were probably never united, even in Twilight’s time. We know that the lighthouse should have the Unity Crystals, but the plans did not come that far. It is possible that the Unity Crystals were intended as a security request for something to happen to the magic. S8 in G4 has revealed that magic is very unusually easily affected and can disappear or become erratic, Cozy Glow almost destroyed Equus as a magical world. And the arrival of the windigos in S9 showed that the emotional impact on magic is sometimes too strong.
It was early commented on “forcing a utopia” on the ponies. This is, in fact, an underlying tendency of the human writers who drafted the script and then built up the lore of the MLP. The worst example is the windigos, which created a very serious controversy because the motto would then be “be kind to each other, or die”. It has been seen time and time again that these make a utopian world obligatory, almost like a vital justification for existence. That kind of attitude was not in G1 or in the G4 IDW comics. Equus should only exist for the kind who can die out if they stop being kind and happy.
This may be the motive behind the Unity Crystals and the disappearance of magic for the authors, who want to emphasize that kindness, union and happiness should be vital in the world of ponies. Yes, even as the very justification for existence. G4 was a success because Faust understood that one must have aspects of darkness, unhappiness and conflict in the worldview of the G4 Equus, the many villains, dissatisfied and antagonistic ponies and non-ponies were important to show a more gray MLP. This was not always respected through G4.
So G5 should get out of this slot as soon as possible, the “be kind or die” setting must go away. Then the instability of the magic associated with the ponies’ feelings and mood must be eliminated. This may be what caused non-ponies to flee, if the pony magic had become a danger to them as well.