Over expansion has wrecked all pro sports. It has diluted the product.
Baseball: Contract 3 teams
Basketball:4 teams
Hockey 4 teams
Football 2 teams
Baseball? Try remove 12 teams! Eliminate inteleague play and playoffs. If 162 games is not enough to determine a league's rep to the World Series, how many are?
Basketball? No contraction, just raise the hoop two feet!
Hockey? Go back to about 16 teams, and go back to a forty game regular season with MAYBE four playoff teams.
Football? No more use of hands by o-linemen, offensive holding 15 yards. No two-tiered facemask running into the kicker penalty... automatic 15 yards. Modifications of most "judgement of intent calls" into "judgement of action" calls. Stick with the 32 teams for now.
I remember Bud Selig trying to contract the Twins and I want to say the Expos not too many years ago. The Twins promptly ran off two or three straight division titles, and since moving to DC, the Nationals/Expos seem to have turned it around.
Two or three teams I would take a very hard look at dumping would be Tampa Bay, Milwaukee, and Kansas City. Prior to winning the World Series a couple of years ago, I would have included the Marlins, but they seem to have gotten over the fire-sale fiasco after their first title and are playing competitive ball. The same can't be said for any of the others mentioned who've suffered from poor management. The Royals would be hard, because they have a rich history, but they haven't been able to afford to field a competitive team in nearly 20 years. Tampa Bay just seems like there's no life in the franchise, and Milwaukee looks the same.
I could care less about the NBA. If I want to watch basketball, I'll watch the college game.
I'd leave the NFL alone for now, but 32 teams is about the max they can handle.
As for the NHL, I'd dump all of the franchises south of the Mason/Dixon line except maybe leaving Dallas and Tampa Bay around. They have the snowbirds to support a hockey team. I'd also get rid of Phoenix and Anaheim. There's just something weird about a hockey team playing in Arizona, and LA doesn't need two hockey teams. I'd give a few other struggling clubs like Pittsburgh and Buffalo 2-3 years to turn things around, or get shut down.
The league just can't support 30 teams, especially those flung around the sunbelt. The fans will turn out for 2-3 years, but eventually, they'll get bored and stop coming. A lot of minor league teams were a big hit down here for a few years, but after the novelty of the fights and complexity of the rules sank in, attendance fell, and the clubs eventually folded.