How deep are you willing to go? The biggest obstacle you yanks have is your religious adherence to the Constitution and your enslaver Founding Daddies. You got to kill of the American Civil Religion, man, it's a cancer.

Beyond that, if I could wave a magic wand here are the reforms I would force on the US political system:

• Massively curtail presidential power, and to do that, actually weaken the federal government, massively. Sorry to sound like a Repubilcan, but it is true that your federalism is too coupled and states are too weak financially because the federal government takes most of the tax money. Canadian federalism is better in this respect: 50%-50%. Let states actually experiment with different economic models.
• Abolish the electoral college and create election standards and procedures, automatic voter registration. Actually create independent election agencies, like Elections Canada. If you need to have each state manage its own affairs, have them all have their own agency, but with each one strictly adhere to the same standard. Voting should be simple and straightforward. No more shennanigans with gerrymandering and arcane laws.
• Massively expand the size of legislatures, ideally also get rid of FPTP for proportional representation. Abolish idiotic things like supermajorities and filibusters. Your legislatures need to be able to function. It cannot be that a few senators can hold 350 million people hostage.
• Massively expand and depoliticize your Supreme Court. 9 justices for life? WTF. There should be term limits, and a pool of justices that rotates. It can't be that everybody hangs from the whims of a couple of geezers.
• Lobbying and PACs and shit, all that is just corruption. Go after it.

Basically, its just Selectorate Theory: the fewer the decision makers and the fewer the people they are answerable to, the more "cosmic" each election becomes and the higher the chance for corruption and therefore dysfunction. Democracy, democracy, democracy. Kill the ghost of Madison and Hamilton. Bury those fuckers forever.