A Wikitravel Expedition is a special sub-project for Wikitravel articles or images. (Sure, we could just call it a "project", but what fun is that?) Expeditions help Wikitravellers collaborate and organize around certain subjects, be they based on shared interests, geography, or shared skills.
If you have a good idea for a new Expedition, here are the steps necessary to organize it.
Make sure that the Expedition complies with Wikitravel's goals. Will the Expedition help us reach our mission of creating a free, complete, up-to-date and reliable world-wide travel guide? Or is it a slippery slope into non-travel-oriented distraction?
Propose the Expedition in the travellers' pub and get some feedback from other Wikitravellers.
Create a page in the Wikitravel Shared namespace called "Name of expedition Expedition" (note capitalization). On that page, give a rationale for why the Expedition is needed, and define the concrete goals of the Expedition.
If additional pages are needed for the Expedition -- for example, template pages -- link to them from the Expedition page.