Pretty big
rule changes coming - minor stuff this year, other than eliminating August trades, but next year:
3-batter minimum (or reaching the end of the inning) for pitchers, other than for injury
Maximum number of pitchers on the roster (actual number to be determined), with every player designated as a pitcher or position player as of their first day on the roster, not to be changed during the season. Non-pitchers can only pitch after the 9th inning or if you're winning or losing by at least 6 runs. (Also two-way players, but it sounds like they have to start as a pitcher until they have 20 innings pitched and 20 games started as a position player with 3 plate appearances, so you'd need them to pitch regularly for awhile before they qualified. Ohtani wouldn't have qualified last year until mid-May).
Roster limit is increased to 26
40 man roster limit in September is eliminated, and teams must carry 28 active players