<p>Besides the subjects already mentioned in the question, I can think of some suggestions to improve the site's statistics (and perhaps indirectly increase the number of participants too):</p>

<p>short term: </p>

<ul>
<li>support <a href="https://meta.stackexchange.com/questions/125740/dont-throw-away-all-votes-when-a-user-is-deleted">this</a> proposal, which asks for votes not being deleted when a user leaves the site.</li>
<li>once in a while, pay attention in the active tab, to questions bumped up by the community user. They are questions with 0 score answers and maybe the answer(s) did not receive enough attention. This <a href="http://data.stackexchange.com/stats/query/252822/unanswered-questions-with-answers" rel="nofollow noreferrer">query</a> written by <a href="https://stackoverflow.com/users/3717023/raff">Raff</a> also clip such questions.</li>
<li>go forward to the <a href="https://stats.stackexchange.com/help/badges/59/announcer">announcer</a>, booster and publicist badges. They help to improve the site's visibility and maybe it will attract more active participants.</li>
</ul>
