We need to open the Wikimedia Hackathon 2015 call for participation. For this, we need check the lessons learned from the Hackathons in Zürich and London, and the MediaWiki Developer Summit in San Francisco. We probably should go for the Phabricator-based system used in SF.
The call for participation is also conditioned by the goals agreed for the Hackathon. For instance, last year in Zürich there was an explicit push for Maps. Last week at the MediaWiki Developer Summit there were three main themes (Mobile, Editing, Services). If we want to push main themes in Lyon, it would be good to agree them at T88520: Goals and measures of success of the Wikimedia Hackathon 2015.
Meanwhile, I have added the current sessions suggested under a "Call For Participation" column in the workboard.
The call for participation is happening, see https://www.mediawiki.org/wiki/Lyon_Hackathon_2015#Topics and the amount of proposals at Wikimedia-Hackathon-2015
This is not a conference but a hackathon, and this is why the call for participation consists of proposing activities and trying to get critical mass to work on them during the event.