After discussing it with @librekitty in the past I think there's a pretty simple recipe for a nostr reddit.
Just let hashtags act as subreddits, and use WoT (possibly with PoW) for moderation. The WoT system could be as simple as just letting people group up into "mod teams" that each user can opt in or out of allowing to filter their feed, like how subreddits have "mod teams."
You could take it further by having a filter mode that only shows content from ephemeral keys.