I like Elon's proposed party platform of a pro-tech, pro-free-speech, and anti-regulation centrist party. It sounds more libertarian than either of the two major parties, at least. But I don't think that most Americans care enough about these policies for it to really catch on unless he can find several ultra-skilled politicians to spread his message. Elon himself is a terrible politician, as well as a nutcase who I definitely wouldn't trust to stick to any particular platform.

I think that the best way to try to create a third party would be to focus on having an unusual and attractive style, more than on any particular policy, and I'd particularly focus on the rare style of being reasonable and nice. Today, there's no point in ever listening to 99% of politicians: when you see a politician from either side, you know that they're going to make big speeches with zero substance, be intellectually dishonest, and trash/dehumanize everyone with any connection to the other side. A party full of politicians who don't do all that would really stand out and succeed, I think. So if I was designing a new party with the main goal of actually being competitive, I'd create the "Compassion Party":
 - All member politicians have to go out of their way to always be nice, respectful, and intellectually honest, even in the face of massive liars/assholes like Elizabeth Warren or Steve Bannon.
 - It'd be especially effective if many of the leading politicians were moderate religious leaders (such as pastors from mainline Christian churches), since they'd have experience and credibility on compassion, and I think that religious messaging is underused except among the minority of religious Americans who are evangelicals.
 - Focus on a few exciting headline policies, such as UBI or major infrastructure projects (eg. building some popular high-speed rail lines). You want to make people pay attention, viewing you as different and likely to actually shift the status quo.
 - On controversial issues, take positions which come off as reasonable, practical, and not ideological. You want to appear "above it all".
 - Once you're actually in power, to ensure that the economy performs well, make hundreds of small technocratic tweaks which are all pro-growth, but are each too small to annoy any special interests too much.
 
That's not my ideal party, to be clear: just what I think would be most effective. I'd rather the Libertarian Party win overwhelmingly and then rapidly shrink the state. Though I do think that the hypothetical party I described would probably be an improvement over the two major parties.