Judging from the posts so far I'm going to get hated a bit, but I'm going to say that aero restrictions would be terrible. Of course, if you think F1 is a show first and then a sport it sounds reasonable, but I refuse to think like that.
F1 is (somehow) still the top of Motorsport, and it is all about going to the limit and being as fast as possible. So what is needed is maximum downforce, and maximum performance everywhere. And also to encourage new ideas, rather than banning anything more intelligent than average.
Let me be clear here, I do not support Mercedes, and I certainly don't like it when they win every race, but if they have the better car and are not cheating that's exactly what they should be doing. Rules shouldn't slow down the best ones.
I do admit that it wouldn't be ideal for TV ratings, but at some point one has to decide what is more important. I always say: If you want equal cars, go watch something with equal cars; leave the pinnacle alone.