Hosting the posts shouldn't be that hard. Storage is so cheap these days. Is it the legal aspects of handling user generated content?
Ranking the posts is another issue. Is that where the value of Reddit lies?
Maybe one could build some hybrid thing which capitalizes on existing structures? I could imagine a frontend which only shows posts by users who signed their posts via their Hacker News accounts. Aka they sign their post with a private key and publish the public key on their HN profile. This way, a new Reddit alternative could benefit from the karma distribution of the best community on the web today.
Hosting the content could maybe be done via one of the new decentralized systems like Mastodon, Nostr or Bluesky? Those inherently have open APIs, so it would be easy to build a frontend which aggregates the content into one simple UI.