I think you could do it by having say a list of hashtags and you click that list and then each post with that hashtag comes up analogous to a community. I think mainly it would be an interface that needs to be written. Not a change in the tech itself.
Discussion
have you ever seen Thunder for lemmy? thats what i would want but for nostr. https://github.com/thunder-app/thunder/tree/develop
the compact view in screenshot 3 is what i like best.