I know there are a bunch of people just picking up this game now so I wanted to share my custom Quickstarts that you may be find fun. To install, unzip and place in your documents/ootp/quick starts folder (PC) or "~/Library/Containers/com.ootpdevelopments.ootp21macqlm/Data/Library/Application Support/Out of the Park Developments/OOTP Baseball 21" (Mac).
World Templates (wtp) go in the world_templates folder and can be loaded after starting a new league or in the custom game menu.
For each save I have disabled facegen & baseball cards along with box scores & game logs for optimal performance. Detail is also set to "Normal" from "Very High." Also some start in the future. Sorry about that. Usually the way it goes is that I set up a world that I am not sure will even work. Then if it does work, I back it up.
MLBfeederNCAAandJUCO.wtp. Probably what you want if you're not interested in High School feeders. NCAA D1-D3 & JUCO D1-D3 feeder into MLB. No HS. Current MLB roster with default settings.
2019MLB_NCAA_HSfeeder.wtp. Exactly as it sounds. HS teams->NCAA D1-D3->MLB. Starts in 2019.
32Team_Ind_Tourn.quick- 32 team MLB expansion with division realignments (8 divisions), along with independent US leagues and an Independent League Tournament set up. Starts in 2021. I prefer using tournaments vs association playoffs because association history is not saved.
Have fun! If you have any roster issues, you'll want to change the Rules->Foreign Players on Active Roster Limit setting from "No Foreigners" to a few foreigners, then run League->Run CPU manager and League->Fill all rosters with fictional players.