Make the Ball Explosive (regardless OF SPORT)
Make the goals robotic, and will move away from the player trying to score.
Swords, spears, knifes and other weapons are mandatory to play.
Forget Ninjas, make everyone a Spartan.;
If another player fouls you, you get to fight them to the death right there at that moment, right on the spot.
No more 9 innings in baseball, make the game rigidly timed like real sports.
In Golf, you must fight your way to each hole, you got to fight a Dragon before you can score at the 18th hole.
All sports involving water will require piranhas or man-eating sharks.
At random intervals hungry Lions, Crocodiles, bears , tigers or dinosaurs are let into the field.
Like in Pinball, a player can trigger a multi-ball.
Full contact sports have the option of playing in 60 foot tall combat mecha.
The stadium/team is not responsible for spectators being killed during a game, actually each team is required to kill random "innocents" (philosophy here is no one is innocent, not even new borns)
Any sport that calls for horses now gets Grizzly bears or raptors instead
On Sudden Death both teams are executed, no ifs no buts, they both lose.
Replace automobile racing with jet fighters
Losing team at championship match gets their home town nuked