Not quite a social network but a new version of the web where documents are just markdown or something very similar and the browser is an unglorified document reader with no JavaScript has been on my mind for a while.
And let’s support JavaScript, but you can turn it off if you’d like.
It would be a pleasant and interesting contrast to the giant anti-user Rube Goldberg machine browsers have turned into (err actually just two browsers both funded by the same company because the nightmareish complexity makes it impossible to do competing browser implementations).
Feel free to call my idea stupid (it is) but let's not pretend HTML and the current implementation of the web is some sort of perfect ultimate gold standard we can't improve upon.