A plethora of mine, some of which are the same or similar to others posted here (I'll acknowledge those if I can remember who said it). Some of my ideas contradict other ones, so this is not a slate, but more like a menu.
All 30 teams make the playoffs every year (credit Mike Sixel)--if you lose in the first round, you get a pick between 9-16 in the upcoming draft; tanking immediately ends.
MLB pays off 20 MiLB franchises and shutters them. 10 teams are added to the "MLB" tier (I'd say Portland, LV, Nashville, Charlotte, San Antonio, Vancouver are for sures), which splits into two leagues, the Gold League and the Silver League. Each league plays 7 Each year, 2 teams are promoted from the Silver League to the Gold, and two relegated from Gold to Silver. Teams promoted cannot be relegated unless they've completed three seasons since their relegation, and teams relegated cannot be promoted unless they've completed two seasons since their relegation.
Players are no longer paid by MLB team, but by the MLBPA, which receives a set % of total MLB revenue. The MLBPA can choose to play players by any metrics they choose, and each team is bound by limits (there would still be team control at the beginning of a player's career, and only so many individuals/contract years can be held by one team at a time)
The rules change to make the game last 8 innings, and there are only 8 fielders and 8 hitters in the lineup. Once all 8 spots have batted 4 times, that team no longer gets to bat, regardless of what inning it is (but they do get a bonus run for every inning they did not finish; for example, the team's 32nd PA ends with 2 outs in the 6th--that team gets 3 bonus runs for the 6th, 7th, and 8th innings. Conversely, a team gets a bonus run for every PA under 32 that doesn't happen for the opponent; for example, a team gets through 8 innings and only faces 29 opposing hitters to get those 24 outs. They get 3 bonus runs for the 30th, 31st, and 32nd hitters that did not appear.