you have a few options for supply players to a league...
draft, leftover fa to be signed from wherever and feeder system... (any others? oh, simply creating x/year instead - option visible when you turn draft off in settings)
you create a large enough feeder system to supply the major league... you'll have to search forums for some ratio ideas... also, whether you want to make 2 - 1 HS one College feeder etc...
since there's no minors at present, i'd model how many you need based on how ootp handles historical eras with no minors.. a very slim draft is required, from what i've read for leagues of a similar setup. so, you wouldn't need a suggested feeder system for a typical MLB 30 team league... it would be way smaller...
# of rounds -> how many teams needed. that's what you'll have to figure out. more layers of MiL at any level means more rounds required, which means more feeder teams required.
extending the simulation beyond MiL and into HS and college player creation. just 1 more layer for the engine to manipulate the players over time. may roughly be an equivalent but also more volatility likey, i'd guess. (of course assuming you don't flood the draft... make millions of players and it will get flat, possibly
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