Unfortunately with today's SPA apps we don't even get the HTML directly, but with the recent resurgence of server-side rendering we may soon be able to get rendered HTML with one HTTP request. And then the only hurdles will be legal.
1. SPA that you can run on your phone or desktop
2. Centralized User Management, need some way to block known bad actors
3. Signing posts / comments
4. Distribution of posts and comments over DHT?
5. Hosting images, videos and lengthy text posts on torrents
6. A whack ton of content moderation software to somehow make decentralized moderation work.
7. Image recognition for gore / CP that inevitably will get spammed
This would enable people to help host the subreddits they are subscribed to, but murder battery life on mobile unfortunately.