We have AA and AAA, but it would be cool if we had a minor league system that was a little more in depth. I don't know how everyone would feel about that, but what are your thoughts?
Absolutely 100% want it added. The reason it wouldn't have been added in the PS4 generation would simply be put down to return on investment. Not perceived as worth it given probably less than 10% of players would see the value.
Hopefully that investment is now required / perceived as desirable to balance things with the roster expansion in MLB and the impact that has had on the single A rosters. Also expanding the market beyond PlayStation may see a whole other group of people interested in non-DD modes.
Prior to PS4 it may have been a system limitation issue (storage and or RAM available).
Licensing also an issue. That's another 90 teams, logos, etc that would need to have rights purchased.