So, makes it kind of like polo, except no riders? I’d watch it at least once.
One other change. There will have to be a three-hour break between periods to resurface the ice.
Forget the crampons, damages the ice. All participants shall wear leather soled dress shoes.
And rather than a small, rubber puck, the game shall be played with a 42-pound piece of polished granite.
Football, baseball, basketball, hockey,…would all be played while the players are riding or at least seated on horses. Horse racing and polo on the other hand…
Basketball: The nets would be moved up to 40’. And no backboards. The court length would be increased to one mile.
Any sport: At least half of the team on the field at any time would have to be people from the stands, picked from the crowd at random.
Horse feces are too much a nuisance.
I suggest unicycles! A player is out of bounds whenever any part of the body is touching the ground.
The ice hellscape is part of the appeal. 
If we’re going with smooth-bottomed leather shoes, I recommend replacing the ice with polished concrete covered by 1-mm layer of mineral oil.
Football - In order to prevent CTE all NFL football is now to be played with flag football rules.
Except for the Super Bowl, that goes back to normal NFL rules because that’s the only game that matters.
But to make it fair, we need to increase the range for a 3-point. I suggest half-court. Exactly half-court, no more, no less.
We can also get rid of that silly goal and replace it with a target on the ice.
Hold on, I thought we were supposed to be ruining the game, not making it better.
Curling could be improved by banning those silly brooms and instead have teams move the rock with small explosive devices.
Bottle rockets?
Used to actually be a thing. Growing up in small-town Minnesota in the 1960s-1970s, there would be the occasional donkey basketball game, sponsored as I recall by the local Lions club. 10 donkeys, a referee (I don’t think he was mounted) and some local popular guy with a broom and shovel on clean up duty in the high school gym.
May have been a baseball/softball version also.
And to think that we weren’t even allowed to wear street shoes in the gym for fear of marking up the floor.
It is apparently still a thing you can do.
Once again I got to listen to my 92 year old father in law* rant this weekend about how a slam-dunk in basketball should only be one point, because “there’s no skill involved.” 
Which got me to taking it a step further - there should be multiple arcs on the basketball court. One would line up with the free throw line - anything inside that arc only counts for one. Between that line and the three point line is two. Then there should be another line halfway between the three point line and half court, which counts for four. Since free throws are behind the one point line, they’d count for two. And finally, if you make a basket from behind the half court line you automatically win the game.
* This is the same guy who thinks a home run in baseball should be an out.
The problem I have with basketball is that fouls are an accepted part of the game in the last minute or so of a close game. I’d fix that by introducing the penalty box from hockey.
You know, I’m not sure that would be so bad. Certainly change the game dramatically, but would still make for a very playable and watchable sport.
There is another thread here about changes to the NFL overtime rules, but my idea is better suited to this thread.
I would like NFL ties to go straight to a shootout.
Five attempts for a field goal from the twenty yard line. No defenders. Only the holder and kicker on the field.
Oh, and only linemen can kick.
Have to get a rejuvenated Lou Groza.