You can think of a game, maybe in a general way and in a very flexible context. We know one of the goals are to collect something. The crunch comes in when they announce the theme and your game conflicts with the theme.

Collectible items are the main theme, for an extra challenge, 3 other limitations are revealed at the start of the jam. So right now you are free to think of any ideas based on the main theme, but if you want to include the extra limitations, you'll have to wait till the jam starts.