If Mercier and Sperber's theory is correct, people are already optimized for arguing things out in groups
Not really, no. People are optimized for winning arguments against untrained humans. The point of group rationality training is figuring out what norms / individual training / etc. makes it so that the best ideas (by some external metric) are most likely to win in a group discussion, rather than the best-championed ideas. Even if, say, I can identify why someone's argument is not helping push towards truth, there needs to be a group norm that I can call them out on that and that will be effective. (Think of "Objection!" or pointing out fallacies in debate club; both of those rest on the common acceptance on what things are worth objecting to or calling fallacious.)
The average person isn't as well optimized at group debate that the best debates, but people are still optimized for group debate in the sense of individual pondering.
As many people have noted, Less Wrong currently isn't receiving as much content as we would like. One way to think about expanding the content is to think about which areas of study deserve more articles written on them.
For example, I expect that sociology has a lot to say about many of our cultural assumptions. It is quite possible that 95% of it is either obvious or junk, but almost all fields have that 5% within them that could be valuable. Another area of study that might be interesting to consider is anthropology. Again this is a field that allows us to step outside of our cultural assumptions.
I don't know anything about media studies, but I imagine that they have some worthwhile things to say about how we the information that we hear is distorted.
What other fields would you like to see some discussion of on Less Wrong?