I'm a big fan of futsal. I think it should be promoted in American soccer circles to train players because the skills do translate to the outdoor game.
My idea is either a season kickoff tournament where young players from academies, draft picks etc get put into a futsal type tournament. Single elimination all MLS teams compete, and you promote it in arenas across the country to promote the game using your young stars It serves as a dual training game since futsal is known to help players with ball skills and playing in tight traffic.
If you don't want to do the preseason tournament, do a Young Stars All-Star game (MLS vs. MLS), or maybe do a play on North America vs. The World with young players. NBA does rookie and sophomore All-Star games during All-Star week. I think it would be a cool way to promote MLS All-Star Week.
Again the All-Star game would be futsal as well but I'd take a young stars outdoor game as well.