“It can’t be… It’s the Princess’ Journal. This journal is the key to finding Equestria!”

Sunny: I had this for ages… Kept it locked away
Sunstreaker: (Gasps over a fond of memories)

I was thinking the same thing!  
This makes me think of something.  
Like what if the world of G4 was a bit like the city of Atlantis?  
Maybe it seems that everything has disappeared for a long time, but instead the G4 world still exists, only that maybe it continues to do so in another dimension where time doesn’t exist or passes more slowly?  
A bit like what happened to the Crystal Empire.  
The Crystal Empire has disappeared for a thousand years, but when it reappeared in season 3 it was all exactly as it was as if all those years had never passed.  
Perhaps the world of the G4 was also hit by a curse similar to the one launched by Sombra on the Crystal Empire and disappeared into thin air leaving the surviving ponies disoriented who in the end decided to found a new Equestria in another place, but then they found themselves in disagreement and they have decided to split again.  
If this were the case, it means that the mane 6 and the rest of their world are trapped somewhere and must be saved.  
Maybe Sunny’s father had discovered this and was looking for a way to bring that world back and this explains why he was so obsessed with his research on the Ancient Equestria, but since he is probably dead, now it’s up to Sunny and his friends to continue his job.  
It would be very adventurous to see the mane 5 go in search of a way to bring that world back and in the end they succeed, thus allowing Sunny to fulfill her father’s dream and it would heartwarming see all the G4 characters come back in the final.

“It’s true all along… Equestria does exist.”