UPDATE 1-20-16: Final standings below, with modified relegation/withdrawals based on 2016 teams.
Just suppose that many years ago, the US Soccer Federation required all professional teams to play in the same league system. Just suppose that they decided that it would be best to always have two equal leagues, east and west, and then divide teams vertically into Division 1, Division 2, etc. Just suppose that, each year, they promoted the best teams from Division 2 and relegated the worst teams from Division 1.
And here's the big leap. Just suppose that we didn't have any more teams playing than what we actually have.
In 2015, the USSF had 55 professional teams. Two teams folded after the 2014 season (Chivas USA and Dayton Dutch Lions). New teams added to Division 2 West were Austin Aztex, Colorado Springs Switchbacks, Portland Timbers 2, Real Monarchs SLC, Seattle Sounders 2, Tulsa Roughnecks, and Vancouver Whitecaps 2. In the East, the new teams in Division 2 were Jacksonville Armada, Louisville City, FC Montreal, New York City, New York Red Bulls II, Saint Louis, and Toronto II.
To accommodate regional travel, Atlanta Silverbacks moved from the West to the East, while recently promoted Columbus Crew entered the West.
Here is a map of the teams. Click the rectangle tab thing in the upper left to get to the list where you can deselect the separate divisions.
Here are the final standings for 2015.
Since a wonderful home-and-away round robin didn't actually happen in each league above, I've substituted that with ranking the teams by points-per-game, including each team's league and cup games actually played in 2015, only counting games against teams in its division (Division 1 or 2). Orange County and Sacramento played each other twice this year, and no one else in Division 1, making those games very important in these standings!
LA Galaxy II are ineligible for promotion since "B" teams are not allowed in Division 1.
At least one team from each Division 1 league will be relegated and at least one team from each Division 2 league will be promoted. The final number will be determined by the number of professional teams in 2016 and the resultant promotions without relegations to even out the divisions. Without adjustments to the number of teams, three would be relegated and three would be promoted. Based on the announced number of teams in 2016, two teams from each division will be relegated and three promoted, as shown above.