I’d LOVE to see promotion/relegation applied to any of the major sports here.
Baseball and Hockey seem like the two sports that could incorporate such a system most easily—they both have very deep minor league systems similar to most Euro leagues.
Football and Basketball would need to develop proper minor league systems—colleges generally take that role now.
I think eventually will happen with soccer. You’ve got the USL which could merge with the MLS, and give us essentially a First and Second Division. I think it will happen within the next few years.