Somewhat unrelated, but I think Reddit-like communities have a better chance to succeed than topic-specific relays. I think relays are slightly harder to understand as a concept and they work better as something which is "transparent" to the user: a mere technical detail most of the time, which the user doesn't have to keep track of. Users shouldn't be expected to manually select relays every time they post. A solution can be that of having a community and hosting it on a relay specific for that community. Alternatively, a relay could fetch and store events with a given hashtag.