Okay having read all the Theros stuff, and I mean all of it, I've realized coming up with monsters might actually be harder then usual because most of Theros' monsters types match up perfectly with MM monsters.
Monster types that will have to be in MOoT would be Archrons, Constructs, some kind of Nyxborn (prediction Nyxborn will be Celestials or a subtype like Titan), Nymphs, Flying Sirens (as opposed to the usual aquatic D&D Sirens), Theros Lamias, Nosden(The Returned), Eidilons, Demigods, Titans, Mythic Monsters (there are three of them), Phoenix, Leonin. Plus maybe specific individual creatures as well beyond the Mythics like Leonin King Brimaz, and monsters variations like different types of Chimeras.