How would you go about setting up such an open source project? What tech stack would you use and how would you set up the product development process?
I think something built on the recently released Freenet 2 has a lot of promise.
But it's all a popularity contest and that isn't about merit. The number one reason things are popular is because they were already popular or other social networking effects.
Reddit has somewhere between 800 million to 1.6 billion accounts with about 50 million active concurrent daily users/bots. Whether human or bot the platform would need to scale to that amount and quickly remove illegal content at a minimum. Add to this the replacement needs to offer compelling reasons for the existing user-base to migrate.