I would think that it isn't too hard to imagine what the universe would be like if all the fundamental dynamical laws were the same, except entropy decreased over time instead of increased.

It would look like our universe in reverse. It would begin from heat death and end in a Big Crunch. Puddles would form into ice cubes. A wave would converge from disparate sources and carry enough energy to eject a penny out of a fountain and into your hand.

I think the difficulty is: can we meaningfully distinguish that sort of universe from our own, or are we just describing the universe we already live in from a different angle? It might be like giving a description of a pencil that starts with its point and ends with its eraser, and then giving a description of an otherwise identical pencil that starts with its eraser and ends with its point. These could just be descriptions of the same pencil.

I suppose there is another ways that you could have a universe in which entropy always decreases. It could start in a state of low entropy instead of with a Big Bang, and just get even lower in entropy from there.