That said, there are no American sports with a promotion/relegation system (probably because those systems evolved organically in Europe from local teams just becoming good and gaining a following, and the American way of having billionaires owning one of a small, fixed set of teams means that they want to protect their investments and wouldn't accept a chance of being relegated and losing their valuable TV money), so it would be interesting if one of them picked it up. From an outsider's perspective, though, I think it'd probably be better to bias towards simplicity instead of trying to fix every problem. E.g. instead of having the "must win 86 games in two/three seasons to make it to the champions league", fix the number of teams in the top league and promote the top of the bottom league and relegate the bottom of the top league. Making the system more complicated to prevent teams from tanking probably isn't a worthwhile tradeoff, since 1) it sucks when teams tank, but it's very exciting when a bad team starts to do very well (e.g. the Bengals at this year's Super Bowl), and 2) that system still doesn't stop the teams at the bottom of the second league from tanking, since there's no punishment for them and they can get better draft picks.
Though my 2c to improve baseball as someone who just started watching football and tried to follow baseball when I was younger:
1. Reduce the number of games. In football, there are so few games that each one matters a lot. In baseball, there are so many that it's hard to be invested.
2. Find a way to make games more fun to watch on TV. Going to a ballpark in person is always a fun, chill experience as long as you bring a friend. That atmosphere doesn't translate when you're watching on TV, so the game itself probably has to be made more exciting in some way (add more time pressure, or let the players take steroids /s).