Trending hashtags section is coming very soon on NostrNet.work. Initially, it will have a simple interface. I will keep improving it and add new features for better discoverability. it will also support third-party providers in future. 
nostr:note18lmn3jaa7cjc0ce3nwl6pzhl3smuvg0xmegn37962gqhlwrjcy9q23g0a5
Discussion
nostr:npub1gzuushllat7pet0ccv9yuhygvc8ldeyhrgxuwg744dn5khnpk3gs3ea5ds has already made some progress in this direction; it was a good starting point. There is more that can be done here. He is also working on an API, which might be available across all major clients.
The goal is to improve discoverability on nostr.