Low level:
We would have powerful APIs built in for things like distributed hash tables, and talking to certificate and domainless devices by their public key instead of a URL, like what FlyWeb would have been.
We would have a standardised and very easy to work with abstraction for what "An account" looks like, which to a user would probably be something like a WebDAV storage space plus a SyncThing-like replication protocol for redundancy plus a distributed MQTT-like messaging system.
Content wise:
You can keep the ads, the tracking, and whatever, just please make endless scrolling and algorithm curation go away.
It's fine for social media news feeds I guess but horrible elsewhere.
We need to be looking at old forums and what they did right, and looking at how we can capture what made old personal sites special without having to actually run a site.
I'm imagining a forum where everyone's initial post has rich theming possibilities, Wiki style linking to your other posts, tagging and heirarchal organization so you can easily browse everything a specific user did, and easy zip file import/export so you can take your content elsewhere if needed.