We want to discuss topics. This is a place to do that.
Simple need, simple solution.
You don't need an extra incentive to make people talk about things if people talking about things is the thing you want. You don't want to incentivize people who don't want to talk about things to be active somewhere you want people to talk about things because then those people will start doing the thing your'e incentivizing them for instead of talk about things.
I personally only want people who want to talk about things here, and don't want people who don't want to talk about things.