I would probably build something that looks like activityPub/mastodon. Now, how you make it take off for mainstream appeal, I can't tell you. Probably make a client with a new feature the entrenched people don't (or even better, can't) offer you.
The benefits of privacy and less monopolistic control are intuitively something many people consider good, but too abstract for a lot of people to make the switch.
What I'm saying those services need, is a feature they don't get anywhere else. An inovation.