What could/should go on a wiki-like tab on each taxon page? Bouteloua is right that there’s no need to recreate Wikipedia or many of the other sources of taxonomic information about species. (Why would we? The Wikipedia page is already included in the “About” tab of many taxon pages.) What iNaturalist needs, that the other sites don’t have, is guides and guidance on how to identify from photographs combined with the other information provided in iNaturalist observations. Visible field marks, common variations within a species, usual habitat, seasonality, parts of the range where similar species co-occur (or don’t), and so on. I imagine there will be different standards about what’s appropriate for different taxonomic groups, based on what the community around each group needs and can contribute. (Birders probably don’t need to start creating yet another field guide. But some insect taxons really need a community-created field guide.)
Notes about how to observe a taxon might also be appropriate. Including notes about interesting behaviour to observe, as well as parts to photograph to make it easier for people IDing. Maybe the default page should have two sections, one for “How to ID”, the other for “How to Observe”?
When I say “Wiki”, I also have in mind all the usual machinery that goes with a wiki page, including an associated “Talk” or “Meta” page for discussion about what’s appropriate for the page, and a “History” page which keeps a complete record of changes to the page (which allows the community to recover from vandalism). I don’t think there’s much point in an editable wiki-like tab without these things.
Tiwane brings up the need for moderation. (Just brainstorming, here.) Taking inspiration from Discourse, perhaps it would make sense to restrict the ability to edit a wiki-guide to those who have identified a reasonable number of observations in that taxon? The threshold would have to be pretty small for some taxons where there aren’t many observations. Beyond that, the current system of being able to flag things to bring them to the attention of curators seems like it should work just as well for community-created content. Maybe the ability to flag a guide-page, or a section of a guide page?