Ideally we would completely restructure the government to have multimember districts and change the Senate.

Within the current structure, we need to implement ranked/scored voting to break the two party system and the implied complete control it has over our government. It's so much easier for big money to control the narrative, control the candidates, and play off extreme polar politics when the voting system makes people choose the "lesser of two evils".

Were I king for a day in the US, and could only do one thing to help America, changing our voting system to some kind of rank/scored system would be it. Ending gerrymandering and Citizens United are also important but honestly less so than this.