MLB rules changes for the 2019 season:

-There will be no trades after July 31. August trade waivers have been eliminated, though players can still be placed on and claimed from outright waivers, as they would throughout the rest of the year.
-All-Star voting will still be conducted by fans online, but the top three players at each position, in each league, will now participate in an All-Star Election Day. -The top three vote-getters at each position, in each league, (top six in the case of outfielders) will receive bonus payments.
-The Home Run Derby will now come with $2.5MM of prize money, including a $1MM prize for the winner.
-The maximum number of mound visits per game will be reduced from six to five.
-Commercial breaks between innings are reduced to two minutes in length for all games.
-The MLB and MLBPA will form a “Joint Committee” to discuss further issues and rule changes.

Effective Beginning in 2020

-The standard roster size in regular season games and postseason games will increase from 25 to 26 players. Beginning on Sept. 1, roster size will expand further to a 28-player maximum (as opposed to the current 40). A maximum number of pitchers will be designated by the Joint Committee. (Passan reported that the league has proposed no more than half a team’s players can be pitchers.)
-Position players are only eligible to pitch in extra innings or when a team is leading or trailing by seven or more runs. Certain position players may be designated as “two-way players,” but to be eligible, they’ll need to have accrued at least 20 innings pitched and started 20 games as a position player/designated hitter in the current season or the preceding season (including at least three trips to the plate in each of those lineup appearances).
-A pitcher must face at least three batters per appearance unless he is removed due to injury or the half-inning in which he is pitching ends before three batters have come to the plate.
-The minimum length of stay for pitchers who are optioned to the minors or placed on the injured list will increase from 10 days to 15 days. This change is still “subject to input” from the newly formed Joint Committee.

Source: https://www.mlbtraderumors.com/2019/03/mlb-mlbpa-agree-single-trade-deadline-expanded-rosters.html