If I'm appointed to fix NASCAR the first thing I do is summon Brian France to my office. Then in my very best Vince McMahon voice, I yell YOU'RE FIRED!!!! Maybe finish him off with a stone cold stunner for good measure.
In all seriousness though, phantom debris cautions would be a good start. Also I'd like to change the schedule around to make more logistical sense, save money too. I'd add a road course to the Chase and take out a cookie cutter.
Also I'd like to do something with the tires, like maybe have multiple compounds available during the race. It leave the CC's the choice as to which ones to run the hard or soft. It could create some more interesting strategies, long runs vs short runs. I'd like to open up the setups a little more, give the CC's more options on how to setup the cars, not sure how to but I feel like they're in too much of a box as to how they setup their cars.