Offseason training camp for stamina only - I've suggested this years ago.
Almost all basketball teams train stamina before the season starts, most do it in special camps.
This would be a nice addition to the game. Maybe with some tiers. 1 (low cost or even free, low training effect), 2 (moderate), 3 (higher), 4 (highest). Costs depend on division/league income. This shouldn't be too powerful, of course, and could be done by the first training in the first week of a new season.
Just my 2 cent.