Seriously, if third parties actually cared then they would contest local elections and try to gain senate/house seats which could force democrats and republicans to work with them to pass legislation.
But when third parties only show up once every four years in the single biggest election it shows that either they’re trying to play spoiler or just doing a quick cash grab.
The difference between the left and the right is that many Republican voters will vote for a candidate based on a single issue, while many Democrat voters will not vote for a candidate based on a single issue.
I’m genuinely not sure if there is a set of policies the left can offer which will adequately appeal to their entire base. I think the best case is another 2008 Obama who didn’t actually campaign on much hard policy other than just idealistic “hope”, which meant that a lot of voters could see what they wanted in him.
Meanwhile Republicans will continue to destroy the country while throwing out scraps like marginally lower taxes (but much lower for the rich).