Through the powers of the Mythical Pokemon Hoopa, you'll enter portals and explore an alternate "Hyperspace" version of the game's setting of Lumiose City. It basically looks like normal Lumiose, but everything is chrome. This version of the city is populated with Pokemon not seen in the base game, including Alolan and Galarian variants of Pokemon from previous titles. Even if you leveled out your party in Legends: Z-A, expect wild Pokemon in Hyperspace Lumiose to provide a challenge in battles--some of them even exceed level 100 with "hyperspace levels."
As we knew from the original announcement, Mega Dimension will include Hoopa, the dimension-jumping legendary Pokémon, opening rifts to what looks like different versions of Lumiose City that include different wild Pokemon. We see a few monsters not native to the Kalos city, including Chimecho, Baxcalibur, and Corviknight in the trailer, the latter two of which are getting new Mega Evolutions as part of the expansion.