The information from the book was that the three crystals that had become “guardians” came from a shooting star. This would automatically mean the Unity Crystals, but how these crystals got into the ponies’ hooves has not been explained. In C3 there are visible connections between Twilight Sparkle and the Wishing Star, and the book was about the Winter Wishday - then it was thought that the shooting star could only be the Wishing Star.
In a couple of episodes in C2, Twilight’s cutie mark was seen on shooting star images. I went further back and look at C1 again. On the glass window in the airship station, where the three crystals are currently separated. And it seemed that the cutie mark is not a symbol of power, but something in the sky. As with the two crystals, the pegasus and the unicorn crystal - they were in halos with rays as if they were also heavenly bodies that had fallen to the ground.
We know from C2 that Twilight Sparkle was behind the Unity Crystals, and that she has left a message - which is still mysterious; how could she know that the spell meant to keep Opaline away was broken - and why is she “with them”, in her message to the Mane 5? We don’t know how the Unity Crystals ended up in the hooves of the royal pegasi family, with Sunny’s family and in Bridlewood.
But the book - and the new interpretation of the glass window - suggests that the three tribes were together after Twilight may have left them. The first generation after her time may have been united to begin with. And the book, which Argyle and Sunny must have undoubtedly been familiar with, has revealed that the guardians had forgotten their purpose. This may have led to the research of the Memory Stone discovered in the light tower in the movie.
It is possible that the MYM team will let the scenario of loss of magic through loss of friendship become canon, but the response from the fans may lead to them moving away from something so stupid even for children. It is possible that they will continue with it, MYM has become a bit too childish, more on the G3 standard than with TVT and IDW, which are on the G4 standard.
There have only been four alicorns with cutie marks in the G4 - Celestia with the sun, Luna with the moon, Twilight with the star and Cadence with the crystal heart. Opaline has said that the alicorns are immortal, but we know that they age with time. And what’s more, these had to go away to make room for new generations.
The hypothesis that Twilight has become the Wishing Star leads one to ponder why the sun and moon move around the planet, which we know is static with no power of movement. An old thought is that Celestia and Luna ascend and become united with the Sun and Moon.
The G4 lore is supposed to be the background for the G5, but while IDW has shown loyalty and TYT seemed to be very steadfast, it’s more difficult with MYM who at times seemed to move towards an AU reality as seen with G1 and G4. We know that the G1 lore from the 1980s does not exist in the G4, and was instead used as a treasure trove for world building in the G4, with success.
If the AU rule must be inprinted in the G5, it will mean a breach between the G4 and the G5. No more kirins. No more changelings. No more lore about the Mane 6, MYM did not follow up the movie about the Mane 6 which remains almost unknown to the Mane 5. No more the G4 Equestria.
So the C4 is now very important to see if the G5 will be a continuation of the G4 or something completely new.