I've long thought that the only way to make a sustainable social media platform is to have it be controlled by its contributors.
Governance is human problem and not a technical one. We have a few millennia of experience organizing and governing human effort, yet the control structure of every subreddit is akin to some kind of Oligarchy with unelected moderators wielding considerable power. These moderators in turn are at the mercy of the Oligarchical Reddit admins.
A better system would be to offer different governance types as a choice for organizing the subreddit. Have different types of built in voting mechanisms if people want to use them but still allow 'dictator' subreddits if people want to use them.
Set up a revenue sharing system where each subreddit gets the portion of money they generate for Reddit. Allow the governance structure the subreddit has decided on to dictate the way those funds are split between moderators and contributors.
Then set up some sort of governance structure that rolls up the Subreddits. As an American I'm biased towards the way the US does things but I'm sure there are other good ways to do this as well. You could have the equivalent to 'states' similar to the sections of a newspaper: Sports, Entertainment, Money, Technology, Travel, etc. with the equivalent of 'counties' within those (football, baseball, basketball or books, video games, movies, etc).
Replace the corporate Reddit structure with the equivalent of a federal government: have the 'states' hold a 'constitutional convention' and delimit powers that the corporation should have and reserve every other right to the individual 'states'. Then you can have some kind of election system that governs the corporation, CEO being equivalent to president, etc.
Then you have a system that broadly aligns the interests of the participants and provides mechanisms for information to be transmitted when needed for decision making.
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The other problem that social media needs to solve is that of anonymity and content presentation. Here I think Reddit fails by having a global karma count instead of one that's localized to the 'state' and subreddit you're on. If they did that then users could actually gain reputation and credibility within the context of a space.
They also should offer identity services that allow the verification of information like employment, citizenship, education, etc. that some subreddits have their own haphazard mechanisms for verifying. This would allow moderators to leverage this type of credential more easily and users to have more trust in sharing them (because they're sharing with Reddit proper not some random subreddit moderator).
Finally, they should have some sort of verification system that lets users tie their accounts to their real identity. Obviously anonymous accounts should still be allowed but this type of verification system tamps down on many of the problems that social media platforms face.
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If anyone wants to fund the development of a platform like this let me know lol