You can make any kind of game you want, any genre, as long as the main focus is on the movement mechanics. Think something tried and true, like grappling or even as simple as driving, is fun? Go for it! Want to do a twist on something basic? That's great, too, uniqueness is encouraged. Have a crazy concept that you're not sure will work? Great, perfect place to test it out! As long as your movement system is fun, and an integral part of the game, it's perfect for the jam.

if you have absolutely no idea, here are a couple I threw together a while ago and never fully used:

https://solarpellets.itch.io/movement-jam-ideas