Damus-only lists, or are you working on a NIP? Would be cool to have #hashtag lists on the protocol level. There’s actually lots of things that deserve lists support.
Discussion
You could do this already using a bookmark list (kind: 30001). Since each list item is just a tag and `t` tags are specifically for hashtags, this would work.
Of course, clients would have to be looking for them for it to be very useful...