In US soccer, pro/rel.
That is a great topic for discussion. I would love to see that happen but I think given how soccer franchises are handled in this country it would be a very difficult task.
There are a lot of articles out there with proposals about how it could happen. What it would look like say for 2020. Etc.
This one is a decent look at a hypothetical 2020 season based upon promotion/relegation from last season. The biggest issue to me is shown in this article and that is the stadiums the teams play in. The promoted teams play in stadiums along the lines of 5000 capacity. That just can't generate enough revenue. So to me even if they could fix the franchise cost issues. Player contracts and labor legal issues. This one is a potential show stopper. You simply can't have a team in the MLS playing in such small venues. If you could figure out short team lease arrangements for other venues perhaps that could work.