There's hardly any leadership or plan for either party, really. Unlike most countries, the US has very weak political parties. Each senator, governor, House member, etc. in a party can have very different policies from each other, and they can do almost whatever they want without fear of sanction from the party, unlike backbenchers in most parliaments. So when change happens in a party's platform, it's because some individual politician hits on something that is especially effective, in an entrepreneurial way, and then everyone else starts copying him; it's not because a committee decides on a new strategy for the party. For the same reason, it's unlikely that either party would ever dissolve, since the members can already just do whatever they want or need to do differently.
Quite possibly, Trump's second term will be a disaster, and the next Democratic candidate won't have to be that great to win, like Biden in 2020. But undoubtedly there will be an actually-popular Democratic president at some point in the future, like Bill Clinton or Obama. It's hard to predict what message they'll use, though.
For a very long time, Democrats have relied on their "rainbow coalition", where they promise tons of ultra-targeted handouts meant to appeal to many different minority groups. But now many ethnic/religious minorities are leaning more and more Republican because these groups tend to be more socially conservative.
Maybe Democrats should try really leaning into their popularity among the college-educated, who make up about half of the electorate. Currently ~60% of college-educated voters lean Democrat, and increasing that to 90% might be possible, since spending 4+ years in a university tends to push you toward a very particular mindset. So forget about any policies that are too divisive on either side or too harmful to any particular group, be as inoffensive as possible to everyone, and focus 100% on being smart, stable, competent technocrats who appeal to college-educated people and many businessmen. Similar to the UK's Conservatives under David Cameron, maybe. (This strategy isn't what I'd like to see, BTW, just an idea for something that might be effective.)
For what I have observed in other countries, when a Government has a strong majority and do not need to balance or compromise, they tend to implement the strongest extreme of their political programmes.
This usually works well in the first stages - there is direction, business know what to expect and some of the changes are probably needed. Some voters get burnt down in the process, but support is so widespread that it is ok.
In the second stage, the "good ideas" have been either done or deemed not possible. People get out of "the high" and start looking at how their lives are still in the same place they were, sometimes worse. More voters realise that their specific problems have not been solved and that many of the promises were not really achievable - at least how they though these were.
From disappointment then comes annoyance from annoyance becomes apathy, as many realise that what they wanted will never be there - no party will ever deliver for them. It only takes around a 10% of people to flip the coin again. Trump "burn" stage will be brutal, disorganised and insulting for both voters and foreign nations. The more radicals will be cheering - but you do not have a 50% of radicals.
Regarding the Democrats, by now they must have figured out that only 10% of the population is gay, only 1 in 10000 are transsexuals and that while them having rights and visibility is perfectly fine, it does not win you over a traditionally socially conservative country to shout all the time about race, gender and everything that makes us different (diverse - and fun in my view).
My guess is that Trump will just hit the brake far too late and will give a chance to Democrats to go back to basics and gain sufficient support. The more radical the Trump agenda, the higher the chance of a reversal.