I suspect that proportional representation is even more important to establishing a multiparty system that forces cooperation rather than polarization.

I would say almost the opposite.

http://asitoughttobe.wordpress.com/2010/07/18/score-voting/a...

But that is a much more significant structural change.

And I'm not convinced "multiparty" is what we want. I think we'd be better off if parties had far less significance. Special interest groups? Sure, they'll exist. People who advocate for, say, bigger vs smaller government, more or less military action abroad, antitrust vs laissez faire, etc. But they don't have to nominate individual candidates...that's really only needed when you have a system that has vote splitting.

That seems like a fine idea to me. How might one actually accomplish it?

Ballot initiatives.

https://www.vox.com/platform/amp/future-perfect/2018/11/15/1...