Football wise there's a few.
1. Offside is offside. None of this active/inactive bollocks. If a player is on the pitch he's affecting play. A defender lying injured on the goalline will play everyone onside as they're deemed active, so why the hell isn't a forward standing beyond the last defender, even if they don't recieve the ball.
2. the rule of 'deliberate' on handball needs to be dropped. Far too open to interpretation. Loads of freekicks/penalties are given where it's clearly not deliberate. However it has affected lay and should therefore be penalised. Not sure how to change it, but at the moment I'm favouring all handballs are considered foul play but are only given as indirect free-kicks even in the area. If someone is deemed to have deliberately handballed it to prevent a scoring opportunity or goal is a sending off.
3. Probably the most controversial but I think the shape of the penalty area is all wrong. If you're on the byline at the edge of the box you've got very little chance of scoring but get fouled and you get a penalty. But just outside the area in the centre of the goal you don't even though you've got a much higher chance of scoring from there. Just seems wrong to me. So I'd have a more trapeziodal shape to it, probably with an arc rather than straight line.
The pedant in me has also felt that a penalty spot should be 10 yards, not 12. On a free kick you have to be 10 yards from the ball, so why should it be different for a penalty?
Agree whole-heartedly with 1 and 2.
3: you could have the pitch round/oval like they do in Aussie Rules (so that they can play in a cricket ground)
4: Adopt the following rules that work well in rugby:-
i. Allow physios onto the pitch to treat injuries while play continues and without waiting to be invited by the ref - improved player safety and flow of the game. And stop this bollocks of requiring anyone who has been treated to leave the field until invited back on by the ref when the ball is live (i.e. a fouled player's team being penalised).
ii. ONLY the captain may approach the referee to ask for confirmation on decisions. Dissent on a decision, kicking the ball away or failure to retreat 10 yards moves a free kick 10 yards forward.
iii. 10 minute sin-bin for yellow card offences (sent off for the second, as currently)
iv. Accurate fixed-time games, with an off-field timer who stops the watch on instruction from the ref for breaks in play, or the second a goal is scored (to prevent lengthy goal celebrations). Match finishes when ball goes dead after 90 minutes clicks over.
Don't get me started on video technology, but VAR sucks and the rugby TMO TENDS to work better.