Last but not least, we have
The Antarctic Research League. The ARL is entirely quarantined from the rest of the baseball world, no free agents come in, and none leave.
This league is where I run experiments on our players such as massive rule changes, tiny rosters, and genetic modification.
Our most recent test subject was Knuckle Arbuckle (originally known as Jim Arbuckle). Knuckle was the worst player in the 2046 ARL draft before our scientists blessed him with a maxed out knuckleball and literally no other attributes.
No actual scientific value is derived from the Antarctic Research League.
(2058 Antarctic Research League Standings)