Each state has an astounding degree of control over their own voting systems for federal elections. Maine and Alaska have chosen RCV in the last decade, and it seems to be working out well for them.

Some states, including New York, allow a candidate to be run simultaneously by several parties, and for voters to then vote for the candidate with a particular party endorsement. This nominally allows a small party to gain status -- say, if the Green Party (tiny) were to nominate Biden (unlikely, but bear with me) and he wins the presidency, then for the purposes of New York State, the Green Party would move closer to being a recognized major party.

The overriding problem is that the Constitution was written with a deliberate ignorance of the concept of parties -- so much so that in the original 1789 document, the runner-up of the Presidential race becomes Vice-President. That didn't work out well; fixed by the 12th amendment.

The other gigantic issue, that has only become clear in the last two decades, is that an astounding amount of the government runs on convention rather than regulation.

Fixing these things would require a significant restructuring of the Constitution, and that opens up doors to... everyone.