Tax code
Simplifying the tax code is a great idea, but right now there is an enormous amount of political power tied up in the government's ability to grant tax favors or punish with tax penalties. Enormous. Almost every line of the code has a special favor bound up in it somewhere. Both political parties are equally up to their ears in this kind of thing--they just have different "customers."
You would never get real tax reform through Congress. You'd get Senate term limits passed first.