Of the top themes, my personal favorite that I feel is most versatile to many types of games is Companion. Basically, all you need to do to fit the theme is have the player/player character have a friend, AI or new acquaintance they are working with. For example, a platformer where you're trying to reach your friend on the other side of the screen. A top down rpg where you found a lost child and are escorting them home. A story game about two friends who are drifting apart. A first person shooter with a drone companion that explains how to play, gives you stats on the current area when you click it and perhaps holds onto your inventory for you.

Versatility is what I consider to be the most important aspect of a theme and Companion definitely has it, though as past jams have shown, many themes can be more versatile than they might first appear. Can't wait to find out what it will be tomorrow.