Cappy is the main gimmick of Super Mario Odyssey. He's a spirit hat (does those exist?) that can take over any form of Mario's literal hats. Once Mario throws Cappy at an opponent, most of the time he can take over their body.
Which allows Mario to take over common enemies from the series like goombas, chain chomps, bullet bills, and various koopas.
Also newer beings like these snow people. The snowman race was fun for what it was.
You could even posses this gliding dinosaur but this wasn't as fun as I had imagined.
One of my favorites in the whole game was becoming the tank. Ultimate power almost there.
But by far my favorite was becoming Bowser after defeating the nemesis. Peach rides on his back to escape the moon dungeon.
And you get to use raw strength to break your way out.
I also thought it was neat there was even a 2D NES mode to Mario-Bowser.