JUST conservation of energy, sure... consider a universe composed of a ball moving at constant speed in random directions.
But conserving everything our physics conserves means you're using our physics. It's not even a hypothetical if you do that.
Suppose you changed electromagnetism to be one over r-cubed instead of r-squared. What conservation law breaks? Or just fiddle with the constants.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous thread is at close to 500 comments.