While keeping the general thread topic in mind, if the US ever entered into a Czechoslovakia - type divorce and the tentative leaders of your new nation asked you to write a constitution for it, what kind of election system would you build?
This is in response to MT's comment on how many things we realistically can't change. So use whatever hypothetical new borders you want and start from scratch. How much would your system look like our current one?
If I was starting from scratch, I would probably have a somewhat more fenced-in government than what we currently have. Something very similar to the bill of rights, along with other restrictions on the central government's ability to meddle in state, local, or individual affairs.
With that done, I'd be pretty open to a parliamentary system. The last two presidents seem like textbook examples of leaders for whom "no confidence" was invented, and we don't have a good way to deal with that. When we elect a lousy president, we're stuck with them for at least four years with no recourse, and that seems bad. I also like the idea of indirect presidential elections, with the president being accountable to the legislature. That would be a helpful corrective to our overly-strong executive branch.
(Also, it is unhealthy for presidential elections to drag on for nearly two years -- parliamentary systems seem to not suffer from that problem quite so much, but we should fix this one even if we stick with the current system.)
As recently as just a few years ago, I would have opposed proportional representation because I didn't want white nationalists, socialists, and other opponents of small-l liberalism to have a voice in government. Unfortunately, that ship has now sailed, so maybe that's worth looking into now.
I'd still have a supreme court with judicial review.