SUGGESTION:
I'm not sure if this was already tested and
failed.

But what about a 'chat-thread'?
Doesn't need to be fancy or realtime.
Just as a place where ppl can casually
have their chit-chat.

No topic-specification.

As soon as a topic hits enough interest,
create a thread exclusively for that
topic and carry the discussion over.

This way, the overall generation of
'dead' threads which might only contain
a question of a single person or so
could potentially be decreased.

Maybe this could even increase engagement
and overall communication between ppl.

Who knows..