The MLS TriLeague:
The MLS TriLeague consists of three single table leagues. Each is a self contained league, but also connects with the other two leagues via interleague play. Each team plays each team in their respective leagues three times. That accounts for 27 games. The remaining 6 games are made up of interleague play. Total season: 34 games.
The top three teams in League 1 qualify for the Champions League, and the top six teams qualify for the League Cup. The bottom three teams are relegated to League 2.
The top three teams win promotion to League 1. The top five teams win qualification into the League Cup, and the bottom three teams are relegated to League 3:
The top three teams are promoted to League 2. The top five teams qualify for the League Cup. The bottom three teams are entered into playoffs against USL teams trying to win promotion the the MLS TriLeauge.
The League Cup basically takes the place of the MLS Playoffs. Teams are sorted geographically into west and east brackets and play home-and-away fixtures with away goals counting as the first tie-breaker. Teams are seeded depending on their standing in their respective league, so if the 1st placed team in League 3 would have advantage over the 2nd placed team in League 1, regardless of their respective league, which creates more intriguing fixtures. Whoever wins the League Cup wins promotion, unless the winner is in League 1. If the team is not a team that would win promotion due to league play, this team knocks the third play promotion winner out of promotion. If the winner of the League Cup is a League 1 team who has not qualified for the Champions League, this team knocks the third place Champions League qualified team out of contention. If the team that wins the League Cup is also slated for relegation, that relegation is nullified and the next worse team is relegated instead. A team that nullifies relegation in this way cannot qualify for the Champions League. Because of these implications the League Cup Final is played the weekend after the conclusion of the MLS TriLeague. The League Cup Final is held on neutral ground.
US Open Cup?
The US Open Cup will not go away. All teams can enter this and there is no qualification process. That's why it's an open cup.
Why This Works:
The whole point of the MLS, as it stands today, is to grow the league and constantly give fans something to cheer about, even if your team has played poorly in the early stages of the regular season. This MLS TriLeague format gives all teams something to play for. No team is out of contention with SOMETHING, and so each league game is meaningful, which promotes a higher level of play. Those who still love the playoff format get it with the League Cup which is worth playing for from any teams point of view. It creates more stakes for teams that do poorly, as well teams more near the top, and the League Cup would be one of the most exciting cup formats I can think of, with so much on the line for teams that may not win promotion or Champions League football any other way.
Please, let me know what you think, as I'm interested how this could playout--even if it never happens.