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 2017, the USSF has 60 professional teams. Four teams folded/withdrew after the 2016 season (Fort Lauderdale Strikers, FC Montreal, Rayo OKC, and Wilmington Hammerheads). New teams added to Division 2 West are Reno 1868 and San Francisco Deltas. In the East, the new teams in Division 2 are Atlanta United and Puerto Rico.
To balance the leagues, Columbus Crew was moved from the West to the East.
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.
Since a wonderful home-and-away round robin won't actually happen in each league above, I will substitute that with ranking the teams by points-per-game, including each team's league and cup games actually played in 2017, only counting games against teams in its division (Division 1 or 2).
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 2018 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.
B teams in Division 2 are ineligible for promotion as B teams can't be in Division 1.