personally I'd prefer a reset, so restarting from the Age of Chaos. We could have a quest in another part of the world, though, and maybe very later on we might eventually get in contact with Gryphus.
Here's an idea though: the Alicorns supposedly had their own land, and Celestia and Luna are from there. I don't know the details, I only know it's mentioned in gen 5. So maybe an alicorn civ quest?
Alternatively... another couple of ideas
1)A quest set right after Discord is defeated, and we control Luna and Celestia (or even just one of the two princesses) having to basically rebuild Equestria from scratch
2)Post season finale: Twilight has now become the princess: Have her actually bring Equestria to the golden age, managing diplomacy and contact with various other polities, advancing culture and technology, dealing with the still deeply racist ponies...
3)Alternative: PRE-Discord. The Three tribes just united: Equestria has been born, but the alicorn princesses are NOT a thing yet. Make Equestria great, actually unite the tribes, and try to prepare for the coming of Discord in a few... I think decades.
4)How about a modern day quest about the Dragonlands? Ember just became the Dragonlord, so now she has to try and rule the dragons. except they're... kind of uncivilized, to put it nicely. But maybe she can change that?
5) Changeling quest: I really like the idea of controlling a hive, with or without a queen. not sure If I'd prefer it in the past or present though.
of course there's all the other races too, and they offer more chances for original developments. Minotaurs, Zebras, Deers, Yaks, Yeti, Abyssians, bird-people (like the parrot pirate from the movie)...