Promotion/Relegation in US Soccer

For instance (Forget current situation), how about each league reserved between 2 and 4 births for promotions.

How this will work?
In the future, MLS could expand to a maximum 30 clubs league members. However the League will have 32 competing teams each season. The remaining 2 spots will be filled with promoted D2 teams.

Each season the D2 and only the D2 teams will compete for a chance in D1 for the next season.

At the end of each season "D2" teams playing in MLS (regardless the performance) will play against the D2 winners and runner-up for that 2 spots.

The same for D2 and D3, however here maybe 4 spots.

So, with this system the team investment to join MLS or other will be secure. However some kind of mobility, Dynamics could be achieved giving soccer some differentiation from other sports. (Don't Forget international competitions, US Open Cup and League System and other elements).

Show Spoiler Hide Spoiler

USL and NASL will be replaced for two new leagues: PSL and ASL.
MLS 32 clubs. PSL (D2) 64 clubs and ASL (D3 128 clubs. B, Reserve, O&O and Affillliated will play in D3 only.
Metro Areas with more than 4M persons could have teams in D1 and D2. Others NOT.
Etc.