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, and then everyone else starts copying him; it's not because a committee decides on a new strategy for the party.

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/policies 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 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 in any way what I'd like to see, BTW, just an idea for something that might be effective.)