I would say jettison the offsides rule completely. Even when it’s being officiated in a way that’s consistent it’s just not a good rule. Basically it outlaws the pass to a breakaway player, and any hockey fan knows passes to the breakaway player are awesome, now you get a player vs goalie duel with help chasing in rapidly for the goalie, very exciting, and often leads to scores.
Making the arena clock “real” would be great, the whole extra time thing is silly.
I also think unlimited substitutions will help. Same structure they have now, 1 player at a time, stoppages only, you don’t want the game to grind to a halt for subs. But with the ability to rest star players you’d get a lot less of the “resting on the field” that happens. More substitutions more often really opened up hockey, because it made the game less about endurance and more about the actual skills of the sport (puck handling, shooting). Right now soccer is a marathon with a ball and goals, unlimited substitutions would make it a ball and goal game with a fair bit of running.
And the unintended consequence would be a bunch of tall guys standing in front of the goal waiting for a pass. And more people of the opinion that soccer is boring.