Given the mixed reception to IDW's "Season 10" comic run and some concept art for a cancelled My Little Pony G4 movie being made available some two months later, this concept came to be.
One idea I initially had was for this to take place in a slightly alternate continuity that would steer closer to what is thought to be Lauren Faust's original vision for the show (i.e. alicorns are referred to as "winged unicorns"). It's complicated as is, and as I'm no professional writer, I don't quite know how to make this work, so I'll leave the timeline be for now.
What is certain is that the story would be darker in nature, kinda like what the crew originally had in mind for the 2017 movie, and it would play up the adventure aspect—again, as Lauren intended. It would also focus more on the Young 6, although Twilight would still play a significant part as she gets the hang of ruling Equestria. The show really did this group of students dirty with regards to development—Ocellus in particular—and from what I've heard, the comics seem to fare better in that department, so no trouble in finding inspiration.
For casting, I'd have most of the original cast reprise their roles, including a few returning faces from the movie. I haven't thought ahead in terms of new roles, so I'll keep you guys up to date as we go along.
I do have one role in mind, and I have to give thanks to alphawp for inspiring me with this choice: Joe Mantegna as the Emperor of Tambelon himself, Grogar!