The Chan Islanders wrote:- Put in proportional representation. A lot of bother would have been saved by that.
- Free the slaves. Might be hard to do, but if I could pull it off I know I'm preventing the entire civil war and all the thousands of lives lost there.
- Clarify or abolish the second amendment
- Clarify separation of church and state. I'd probably get that past through inglorious fear mongering about Catholics or Quakers, but would be very worth it in the long term.
Proportional representation wasn't really a thing back then and wouldn't have gotten much support. Ending slavery in 1789 would have been nearly impossible.