I would change a couple of penalty costs.
First, I'd change pass interference to a 15-yard penalty. This spot-foul thing can really suck when teams look for it on 3rd or 4th and 20-something, especially when it's a close call.
I'd also consider changing defensive holding so it's no longer an automatic first down. I can understand the rationale for it, but so many of the calls are ticky-tack that it can just destroy a team's morale when t's 3rd and 15 and some minor or invisible hold on the opposite side of the play ruins a defensive stop. Hell, make it 10 yards instead of 5 if you have to.