I don’t see any problem with this from a rules perspective.
We’re encouraging games that are just quick and complete slices of gameplay with minimum of fuss in the front end.
I’d suggest that if you wanted to add more gameplay modes and features, then maybe your time would be better spent iterating on the core during the jam, then rolling the extras into a more feature-rich “post-jam” version of the game.
That’s just a recommendation though. So long as a version of the game that follows the 20 second theme is what is presented to players by default when they launch the game there’s no issues with whatever else you want to do