I'd add elements from previous generations, while mixing it up with my own ideas (toned down, of course, since my imagination can get really dark).
Although, I can't decide whether or not Megan should be in it. If I added her to my hypothetical reboot, then Earth would be real, too, and Equestria/Ponyland/Whatever would exist in another dimension accessed by a portal under a tree in Megan's backyard.
There would probably be humans in the pony realm, also, living in a medieval fantasy setting but with bits of steampunk, Renaissance, and magically-powered machinery thrown in for flavor. (they'd ride two-legged ostrich-type creatures as mounts to avoid any implications).
Twilight Sparkle would be the main pony character, and either be Celestia's student, again, or be a powerful sorceress a princess of her tribe
The Moochick would be in it, serving as a wise sage and one of the pony realm's most powerful magic users, battling against Tirek in a great war that happened centuries ago.
There would be goblins, hobgoblins, trolls, ogres, golems, and maybe dwarves, elves, gnomes, and other mythological beings.
If Megan were to be in my hypothetical reboot, I'd probably make her a teenager. Plus, her first encounter with the ponies will be when she bumps into Twilight and one of the other main pony characters (probably Applejack), who are being chased by Tirek's henchmen.You can probably imagine Megan's reaction when Twilight first speaks.
Rainbow Dash would be a Robin Hood-esque thief, stealing from Tirek and giving to the oppressed, accompanied by Pinkie Pie and Fluttershy.
Spike would be an orphaned baby dragon adopted by Twilight's tribe after his dragon clan was slaughtered (off screen) and enslaved by Tirek's forces.
Rarity would be a tailor in Twilight's tribe, and also part-time treasure hunter.
There would be anthropomorphic animals.
Sunset Shimmer would be a unicorn pony disguised as a human in Megan's world named Sarah Summers.
The Sirens would be among Tirek's most loyal servants.
Queen Chrysalis and the Changelings would serve Tirek's empire as spies, but Chrysalis would secretly plot to overthrow Tirek.