Here's a very unique approach for a topic-based client. It also showcases that you can build anything on top of Nostr:

It utilizes nostr lists to save the topics a user has followed.

When a user follows a topic like #cordcutters, it utilizes rsslay.nostr.moe to create a Nostr profile. This profile is generated from the RSS feed of the corresponding subreddit (/r/cordcutters) and includes the #cordcutters hashtag in every post.

Here's an example Nostr profile: nostr:npub123xykft0hzerzpc7chwnqcgpsjl0ws2xs6zrkuyrpgnclgey5a7s2052xp.

Client website: https://agorasocial.app

It is developed by nostr:npub1k979np6dcpwh7mkfwk7wq3msezml48fh7wksp9hakakf8pwk3y5qhdz7te