Change one thing about a sport to make it absurdly dangerous.

[InfinityCubed]

Example: Bowling.

Replace rolling balls with hacking the valves off compressed air cylinders.

 

[Queen Michael]

I choose American football. Velociraptors on the field.

 

[shootthebandit]

Replace paintballs/airsoft with live ammo and add some banger cars. I call it the thunderdome (named after mad max obvs)

Would be a great spectator sport

 

[Terminate421]

I choose any game involving a ball. The ball is replaced by a porcupine.

 

[Private Custard]

Landmine football (soccer for you Yanks!).

Let's see them diving on a field where the risks far outweigh the gains!

 

[Black Dream]

Water Polo. Remove water. Add fire.

Also works for diving, synchronized swimming, surfing and just about any other water sport.

 

[Esotera]

Everyone playing tag rugby has a revolver attached to their head through some intricate mechanism, with 1/6 chambers randomly containing a bullet. Pulling the tag of the side of someone causes the gun to fire whatever's in the chamber.

Serves them right for not playing proper rugby

 

[shootthebandit]

Formula one where the tyres at deliberately designed to explode during the race....oh wait that ones already been done

Golf where the golf course is littered with alligators...weve got that one too

How about we take MMA/cage fighting and we add lions and instead of a cage we have a massive outdoor arena and we give them random weapons too....weve had that one too but it was a while ago perhaps its time to re-introduce it

 

[The Artificially Prolonged]

Hurdling - all hurdles have spinning blades attached.
Sprinting - instead of a starting gun the competitors instead have to outrun the gun.
Relay - the baton is on fire
Bull Fighting - Replace the bull with a tiger

 

[Drakane]

Bull Fighting - Instead of having scores of the Matadors underlings tiring and bleeding him out before ever even stepping into the arena, lets make it a 1 0n 1 match... I bet the bull would win a lot more often.

 

[tilmoph]

Baseball; all of the bases are landmine with a randomly positioned pressure plate. Touch it the wrong way and KABOOM!

 

[Jolly Co-operator]

Frisbee Golf: Instead of Frisbee's, we use chakrams.

Golf: Instead of a club, they use the Ulapool Caber from Team Fortress 2.

 

[TheRightToArmBears]

Football. The goalkeepers are tigers that have been starved for a few days. To keep it possible, they're chained to the posts, but the length changes randomly over the course of the game to keep the players on their toes.

 

[BrassButtons]

Baseball: the ball is a hand grenade

Volleyball: the ball is a bowling ball

European Football: the ball is a cannonball

American Football: the ball is a beehive

 

[Headsprouter]

Boxing, but with spiked knuckles instead of gloves!

EDIT: Actually, let's have the boxers wear sandals, and the floor is a bed of nails.

 

[Dinwatr]

Football: Give all the players except the QBs 3' long rattan sticks.

Fencing: Use rapiers.

Track and field: replace any hurdle or other thing that you're required to jump over with a lion.

Shot put: Have two people throw at a time. At each other.

Rowing: 4-pounder cannons at the prows of the boats.

I've always wanted to do a live-action chess game in the SCA, where the winner isn't the piece that moved into the space but whoever wins the duel.

 

[Lt._nefarious]

Foosball (or soccer): The ball randomly sprays strong sulphuric acid in a random direction

Volleyball (only applies when played in sand): The sand is replaced by shattered glass

 

[Abomination]

Rugby Union & League - spear tackles and high tackles are allowed.

 

[Hero in a half shell]

Javelin: Once you've thrown your javelin you must go and stand where it landed to mark it's position.

Tossing the caber: Instead of throwing a log you must throw the other competitors.

 

[Easton Dark]

American Football:

Yard lines are now laser beams. Tackling should now be considered very carefully.