If the dragons were extinct, the egg would not have been there. Furthermore, we see that the hatching of the egg had happened after the Mane 5 had completed its adventure that had to do with the earth pony magic. And we know from G4 that the dragons and the ponies have not been close to each other, but the magic of the earth ponies is connected to the earth. And the dragons are likewise closely connected with the earth. It may have been the earth pony magic through Hitch that hatched the egg.
And from what was seen in 1.33, Hitch was surprised that he could talk to animals and understand them. Which suggests that the magic may have completely returned to all earth ponies.
But it raises an important question; why is the magic of the G5 unstable? It seems that the earth ponies, unlike the others, had not been given magical powers, and that emotional fluctuations among the ponies have become much more fatal than in G4. There is obviously a complicated story here about the active magic of the world. When the magic returned, we saw how the horns and wings lit up, but not the hooves of the earth ponies.
And since the hole appeared to be a dimensional portal, it may mean that the idea of a dimension that accommodated the magic of ponies as the unity crystal may have been confirmed. And by jumping down there, Sunny may have taken the first steps toward what it meant to be an alicorn. The rainbow that rose from there may be the magic that was “released” as soon as the corruption had stopped.
The balance is restored - but the point of this is that Sunny and her friends have just started, and that the situation around magic is not rosy. It is possible that the magic will eventually stabilize through the G5, while Sunny learns what an alicorn is, and how the ponies adapt to a new reality.
I do not think we will see antagonists here, either. But there may be important hints that do not appear in the trailer. The Mane 5 has only just seen the surface of the unity crystal, and they have learned that there are much greater depths. It’s a sequel to the movie on the exact same track. We will possibly see the antagonists in the 3D series and maybe the IDW comics.