Seems like a million years since I last ran a theme league, so...
The 90M 5 Year Window AL/NL League
AAA: 6/2 Below Average
Pick a 5 year span (the span must start or end with a 5 or 0.)
1900 - 1904 / 1905 - 1909 / 1911 - 1915 / 1916 - 1920 as examples
Pick a league AL or NL.
Pick a team consisting of player seasons from that time span and that league (partial/combined seasons ok... no clones)
Pick a stadium used by a team in that league during that time span (not exclusive)
Once a League/Time Span is taken it is exclusive (ie only one AL 1925 - 1929 team will be allowed, but another owner could have AL 1926-1930 or NL 1925 - 1929).
In the end there will be 12 AL and 12 NL teams... and they will be split into leagues accordingly.
I'll pick my years/league/stadium once 12 teams have entered.