People are struggling with ideas and bugs, so here are some ideas and systems that are buggy and fun:

Gravity

Teleportation

2D Collision

Merging Objects

Portals (Send you to the wrong place?)

Flying

Underwater

Please add any ideas in the comments, we gotta help each other out on this one.

What might help is to think of just one mechanic that you find fun, and then think of a way to break that mechanic in a way that's interesting.

For example, jumping is a fun mechanic, but what if when you jumped you didn't fall back down? Or you jump in the wrong direction? Or every time you jump your top move speed increases for some reason? etc.

Good luck everyone! And remember it's not a bug, it's a 'feature'