There have been 2 open-world Mario games in the past: Super Mario 64 and Super Mario Sunshine. They were both AMAZING, and the technology involved in this one is better, so this is likely to be the best of the lot!
We didn't ever think Mario's hat needed to come to life, but now that it has, it feels so NECESSARY! Your hat is now Cappy, who is alive and can possess other people and objects in order to use their abilities.
Mario shows up in a whole bunch of different kingdoms, which range from super-realistic, incredibly busy cities to epic fantasy environments with monsters and dinosaurs and all sorts of amazingness in them!
As the game has been made by Nintendo, who also made the Switch, it uses it to its full advantage – the whole hat-throwing thing came out of playing with the Switch's Joy-Con controllers and working out what the most fun thing to do with it was. And there's loads of Amiibo functionality, ranging from unlocking extra outfits to secret clues!
Co-op play looks like it'll be great, with one person being Mario and one being Cappy, but there's also a really cool in-game camera system – you can take photos within Mario's world, and customise them with stickers and filters and all sorts of cool stuff!