I come from an area where they’ve been playing soccer instead of football as the fall high-school sport for at least sixty years. Here are the two biggest problems with professional soccer that we don’t see at the high school level that leave me cold: “flopping” and other fakery on the field, and playing for two hours to a 0-0 tie, then deciding the game on 5 penalty shots.
I would also like to note that hockey and soccer are basically the same game, but hockey does not have these problems. Indeed, I’d say make it more like hockey: when you call a penalty on a guy, put him in a penalty box for five minutes or whatever. Two penalties, two guys in the box.
The one change I would make is instead of PKs, keep the game going but with unlimited substitutions, so players don’t get worn out.
Note that hockey games are now decided by penalty shots. (Never understood why we needed to legislate draw results out of the game).