You are supposed to ruin the sport, not make it better.
Football(soccer): abandon off-side. There are sometimes calls for it to increase scoring, but it would utterly ruin the sport.
There are too many possibilities and directions one could take if they wanted to ruin a sport. A more interesting question would be: in what ways would you change the regulations of a sport to improve that sport, with the caveat that the change(s) should also have a good chance of broad public acceptance?
Anyhow, in keeping with the spirit of the OP, I’ll list some “great ideas” I came up with over the years, which have nary a chance in hell of being implemented because society is not yet ready for such wondrous geniusness:
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Basketball - a player is allowed to goaltend if their height is ≤ 1.65 meters. This way shorter players will have a place in the game, and they’d have to make pretty impressive jumps in order to do so as well
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Baseball - Cap the number of intentional walks allowed to 2 per team per game. Each additional intentional walk will result in every player on base advancing another base. If no one is currently on base during the excess intentional walk, then the batter is walked to 2nd base
- To prevent a pitcher from throwing four straight sinkers into the dirt as a loophole around this rule, pitchers are required to throw at least one strike in that plate appearance before they can issue the 4th ball, otherwise the walk will still be considered intentional
- To prevent a pitcher from throwing four straight sinkers into the dirt as a loophole around this rule, pitchers are required to throw at least one strike in that plate appearance before they can issue the 4th ball, otherwise the walk will still be considered intentional
- American Football - if a defense prevents the opposing team from scoring during the opposing team’s possession (3 and out), the defending team is awarded 1 point. Sacks, interceptions, and recovered fumbles are worth 2 points each for the defending team. This way a team can get some points on the board even if their offense isn’t doing jack ****
Any contact sport + blindfolds.
In American Football (which we in Finland usually call armpitball) use only one set of players. No switching players between offense and defense.
In all seriousness, if I were a pro wrestling promoter, I’d have instant replay and a second referee at ringside who could immediately call for the bell if the main ref took a bump or the heel did something illegal while the main ref was distracted. Just because it’s not a real competition doesn’t mean wrestling has to keep using the same tropes that made sense in the '30s - it should still look like a real sport where the officials behave with common sense and there are procedures in place to prevent shenanigans.
Back in college I remember a group of us brainstorming ways to improve golf and one of my ideas was “full contact night golf”.
Not sure if this ruins or helps, but I recall someone saying that Basketball games should only be played with the score tied at 100 apiece and only with the last 2 minutes remaining on the clock.
American football: No pads. No helmets. No clothing whatsoever.
Hockey - square corners
Golf - defense
Cricket - make it understandable to Americans 
This does happen in real life in high schools in smaller towns. They just don’t have enough players to put together a separate offense and defense.
If I finally spring for Out of the Park Baseball (I have some other games ahead on my docket right now) I’ll design a huge park with massively distant fences, get 4 fleet footed outfielders (with all that real estate out there I`ll need them), and basically outrun the opposition. There has been an opportunity for an enterprising team to do this (in a landscape dominated by ponderous sluggers), but no team wants to have bleacher seats half a parsec from the infield.
Golf: goal is to get the highest score
Tennis: only use ping-pong paddles
Hockey: puck replaced by curling rock
Volleyball: 43 players allowed on each side (as per 43 man squeamish??)
Basketball: no bouncing
Baseball: game made about as exciting by continuing for eighteen innings
Mandate clubs be 10x heavier and or have rugby ball shaped balls.
ETA: The latter would also work for baseball.
I’d watch blindfolded boxers.
But don’t change the hole dimensions. 
For Baseball, there is a simple way to make the game totally unplayable, because every single player in the league will be disqualified.:
Make a rule that nobody is allowed to spit in public.
And the world will be a better place.
To ruin soccer(or make it better):
Tom Corbett Space Cadet:Stand By For Mars!
“Mercuryball is pretty much like the old game of soccer,” explained Houseman. “But inside the ball is a smaller ball filled with mercury, making it take crazy dips and turns. You have to be pretty fast even to touch it.”
Which could also work for basketball, volleyball or tennis.
Fencing: Before every pass, each fencer must close their eyes and spin around ten times. Extra points awarded for sneak attacks.
Ponce de Leon Park in Atlanta, home of the Atlanta Crackers in the early 20th century, had a giant magnolia in the middle of center field. The tree was in play.
The park is long gone - it’s a strip mall now - but the tree is still there.
I played about 10 years of this. Pop Warner through High School. Never knew any better. We had 12 players, and Shane always sat on the bench. He wore his pants under the football pants. Weird.