Something I thought about long ago was that it would be better/easier to divide user accounts into "personas": different sets of public-facing IDs, settings etc.

This could be done at every level: the operating system, the browser, websites..

So if you don't care about the website knowing it's the same person, instead of having multiple user accounts on HN, Reddit, you could log into a single account, then choose from a set of different usernames each with their own post history, karma, etc.

If you want to have different usernames on each website, switch the browser persona.

At the OS level, people could have different "decoy" personas if they're at risk of state/partner spying or wrench-based decryption, and so on.