In a report published today by Sports Illustrated's Brian Straus (link), next season when MLS contracts and expands to twenty teams it will decide its champion with a 12 team playoff format. The new format will see two wildcard games featuring teams ranked third through sixth, and those wildcard winners facing the top two seeded teams (same format the NFL has used since the 90s). Under the current format, ten teams qualify with top top three getting a bye and the top seed in each conference getting the Wildcard winner.
Via @BrianStraus: percent of teams to make playoffs: MLS in 2015: 60% MLS in 2020: 50% NBA/NHL: 53% NFL: 37.5% MLB: 33%— Paul Carr (@PCarrESPN) November 29, 2014
The main criticism of the changes described in this report is the further dilution of the regular season. Galaxy fans know all too well the kinds of runs a team can have when they get home at the end of the regular season, as such a run carried them into MLS Cup 2012 which they ended up hosting despite playing in the Wildcard Game.
When Major League Soccer contracted to ten teams in 2003, they stuck with a eight team playoff format which saw only two teams fail to qualify for playoff action. That was the standard through much of expansion until the Wildcard Game was added and single table seeding was eliminated. This solved the quirk which saw the Colorado Rapids win their first MLS Cup as the Eastern Conference third seed, but left open the door that saw LA host MLS Cup 2012 against the lowest seed in the tournament.
2012 does stand an anomaly, the only season in which a Wildcard seed has reached MLS Cup. In two other instances the Wildcard winner was able to advance to the Conference final, so half of the Wildcard winners since the rule was adopted have managed to win in the next round. That's a coin flip on whether a conference's regular season champion makes the Conference Championship series.