Continuing on your suggestion.

There could be open source tooling to create custom private "closednets", with

- trust ring mechanism to allow invitations, flagging, banning, and banning those that invite people who were banned

- the rules of the closednet

- search engine with opt-in scraping

- portal (remember the 80s?) with all the registered nodes, perhaps by service category such as public git repo hosts, web sites etc.

etc.

The first closednet could be Hacker News.