Spend 20 minutes reading this paper.
“Using interrupted time series analysis and regression discontinuity design, we observe an abrupt and significant increase in social reciprocity after the adoption of a threaded interface.”
In short, adding threaded discussions (expanded all the way, i.e. you could have comments indented significantly as people respond to middle comments in the discussion), yielded an increase in responses.
Almost all community platforms today offer threaded discussions. Your mileage will vary, but testing flat, threaded, one-deep, and full branch structures of discussions is a low-cost and potentially high-reward endeavour.