According to multiple Twitter users like Nibel, and the official Assassin's Creed Twitter page, "Assassin's Creed Mirage is the next Assassin's Creed game."
The folks at Ubisoft are excited and can't wait to tell you more on September 10 at Ubisoft Forward: 9PM CEST | 12PM PT. As for what platforms the game will be on, I'm gonna take a guess and say PS4, PS5, Xbox One, Xbox Series X|S, and - if we're "lucky", Nintendo Switch via the Cloud. (Hey, Japan got Assassin's Creed: Odyssey via that way!) Take that guess with a grain of salt, though.
The official artwork (featured below) for Assassin's Creed Mirage shows what appears to be Valhalla's Basim in a hooded robe doing a... assassination.
Fun Fact: Assassin's Creed Mirage was originally reported with the codename "Rift," and the focus of the game will apparently be a smaller, stealth-based title in line with the original games in the series, when there were little-to-no RPG elements in the series. Stay tuned for more coverage on the upcoming Assassin's Creed Mirage, the future of the series, and all things gaming on this blog. See you soon!