A hard salary floor and cap. Super cheap teams like the current Rockies that are all but guaranteed to lose is a detriment to the competitiveness of the entire league, as are pay-to-win juggernauts
No deferred money contracts. They’re bad for competitiveness (see the current Dodgers) and a bummer for fan bases when the time comes to pay out the deferred money and the team can’t afford a viable roster.
I would implement two salary rules for baseball: