Perhaps 1960would be a good cutoff, pre expansion(Angels/Twins in '61, Mets, Colt 45"s/Astros '62)....NL would be 8 teams, most are very deep in talent 20"s to 60"s... Braves, Cubs, Reds, Giants, Dodgers, Pirates, Phillies and Cardinals
AL would be 8 teams, most have very deep talent pools...Yankees, Red Sox, White Sox, Tigers, Indians, A's, Browns/Orioles and Senators...
The 12 team league idea has merit, there would be teams not in use. A 16 team league would work too, just that all teams would be in use every season....either would be good.