I think there are a few things that would help.

Circles are a significant part of that, but also 'hats'. A person wearing the Lego hat is distinct from the same person wearing the fantasy console fan hat. Circles and hats combine so that you have both control over the context of what you say and who you say it to. Circles should also have content types. Twitter, Tiktok, Instagram etc. are fundamentally the same thing with a different content type. It should be possible to have an underlying network to handle arbitrary content as blobs of data, and have the users define the circle content themselves. This may mean you need to download a custom program to view some circle types but if you are not in that circle you can safely ignore the type altogether.

The other aspect is discoverability. Algorithms boosting content are prone to abuse. I think there is scope for human curation in a social network context. We have all seen the obviously broken thing that a user has had no success at finding a remedy though 'official' channels, but once it becomes a trending issue on social media there is a prompt fix. Winning the trending lottery is how information gets to where it needs to be and like lotteries there are far more losers than winners.

I would like to see some sort of curation tree (or more likely feed forward network). Anyone can play the role of a curator and anyone can suggest things to any curator, but curators have the tools to filter and prioritize topics they care about and people they trust. Following curators gets you what you want, from cute cat pictures, to a list of Android Apps containing malware. A key aspect of this that makes it more network like than tree like is there should not be any topic overlord, there may be five Android Malware curators and a thousand cat pic curators. It should be possible to follow all of Android Malware #153 and anything that any three of the others agree on (or any other combination/weighting). As a user with a piece of information, you could just shout at the top of the tree (as may people without a sense of perspective might), which is why filtering is necessary, but in general it is better to tell someone lower down who receives a lower volume of notifications who can then use their judgement to feed information to other curators.

The curation model itself could be implemented within the scope of Circles and Hats with user interface design to suggest the curation model. To operate this it would require significant volunteer effort, but I think that can be achieved. Wikipedia is proof that there are people out there prepared to work to enable better information for people they have never met. I also think that it is human nature to want to tell someone about something that they have noticed. It is why people post to Sites like HN, Reddit, Slashdot

I don't think such a system can be supported by Advertising or Subscription. Advertising corrupts everything it touches and Subscription just closes the gate to the majority of users. I think it can only really be done by donation of time and resources by those who feel like the existence of the network is a societal good.

That last aspect represents a real challenge because in conjunction with the features described above it has to do all of the baseline things that a social media platform. Security, encryption, scalability. I have enough expertise to know what a daunting task this is, not nearly enough to implement it.