In my view, you're asking the wrong question. The major contributors are doing great; they have attracted their own audiences. A better question might be: how can LW grow promising new posters in to future major contributors (who may later migrate off the platform)?

I had some ideas that don't require changing the LW source that I'll now create polls for:

Should Less Wrong encourage readers to write appreciative private messages for posts that they like?

Should we add something to the FAQ about how having people tear your ideas apart is normal and expected behavior and not necessarily a sign that you're doing anything wrong?

Should we add something to the FAQ encouraging people to use smiley faces when they write critical comments? (Smiley faces take up very little space, so don't affect the signal-to-noise-ratio much, and help reinforce the idea that criticism is normal and expected. The FAQ could explain this.)

We could start testing these ideas informally ASAP, make a FAQ change if polls are bullish on the ideas, and then announce them more broadly in a Discussion post if they seem to be working well. To keep track of how the ideas seem to be working out, people could post their experiences with them in this subthread.