I wanna see some more of Tartarus. Pull some more things from Greek mythology, and really flesh it out. Show us more monsters from it (they mention in the show that there are other monsters who live in Tartarus).
Maybe, to make things more interesting, have some Lovecraftian monsters live in Tartarus, mixed in with the Greek ones. I would make a Cthulhu-like monster that dwells in Tartarus's deepest pits, and eventually, he becomes free and reigns havoc and terror over Equestria. He would definitely give the Mane 6 a run for their money!