I'd change overtime for sure. I hate that BS of luck of the coin flip often paying far to great a factor in who wins and loses. Each team has to get a possession.
15 yards for all PI. No more spot of the foul, crap, that gives people a first and goal at the one when their receiver is actually mugging the defender but the ref blows the call and calls the PI on the defender. 15 yards, period. No more.
Get rid of some of that ticky tacky crap with the QBs. A hand brushes the helmet and it's a penalty? Geez. Pathetic.
I'd also get rid of stupid rules that clearly don't apply to every part of the game.
Examples:
#1: Intentional grounding. Ok...so a QB can't throw it into the dirt to avoid a sack but they can just run up to the line and throw it into the ground to stop the clock? What's the difference? Absolutely nothing. You're not attempting to complete a pass you're chucking it into the ground. Get rid of this rule if it's not going to apply to all plays and parts of the game.
#2: The ball breaking the plane being a TD. I know lots of people love this rule but it's stupid and it bugs me. A runningback, or QB, can simply place the ball over the line, without any of their feet actually getting into the endzone, or any other parts of their body for that matter, and it's a TD but a receiver who goes up for a jump ball, in the endzone, and catches the ball, the ball has clearly broke the plane at that point, but they actually have to get their feet in bounds or it doesn't count. That's contridictory (probably spelled that wrong) IMO and it just bugs me. Either the player actually has to score, as in the player actually has to get the vast majority of their body into the endzone, or all players only have to possess the ball at some point when it's broken the plane of the endzone. Not both ways.