User stories:

As an organizer, I want to be able to specify which wikis my event covers, so that people who contribute in that wiki can join with minimal confusion and so I can easily track and remember my events in my own management processes.

As a contributor, I want to know which wikis an event focuses on, so that I can join events in which I can communicate with the organizers and other participants and/or so that I can contribute content in the language or focus area of the wiki.

As a product or data analyst at the Wikimedia Foundation, I want to know the wiki(s) of an event that uses Event Registration so that I can more easily generate data related to event impact per wiki.

User problems:

• The Collaboration List (Special:AllEvents) is global in the Events tab. This is useful in some cases, but it also creates some problems. The main problem is that there is no easy way to know which event is for which wikis. This can make it hard for some people to know which events to sign up for.
• Organizers have a global page to see a record of all past events they organized (Special:MyEvents). Like Special:AllEvents, it is useful in some cases for it to be global, but it can be confusing in other cases if they do not know which event was for which wiki.

How this could work:

• We can ask organizers when they configure registration to specify the wiki(s) of the event, if any
• We can display the wikis (or at least a partial list of wikis, if long) on Special:AllEvents and Special:MyEvents
• We can have search filters in Special:AllEvents and Special:MyEvents to find events by wiki(s)

Some things to consider:

• An event can be focused on multiple wikis. For example, there may be an event for Latin American contributors that is on both Spanish & Portuguese Wikipedia.
• An event can focus on a wiki and be about contributing OR not. For example, an event can focus on adding content to a wiki (like an editathon) or it can be an a community meeting for an affiliate (for example, the Italian user group, which may primarily focus on contributions on Italian Wikipedia, Italian Wikisource, etc).
• We may want to have a restriction on a maximum number of wikis added. For example, is it really useful if an organizer adds 300 wikis? How do we even display that much data in a meaningful way? For these reasons, we have proposed having a 100-wiki limit in past team meetings. This number seems large enough to cover most realistic use cases.
• When we expand the Event List to include WikiProjects (so it becomes the Collaboration List), we will not need to ask organizers for the wiki of the WikiProject, since we will have that data on Wikidata.

Past work:

• You can find some design concepts developed by @gonyeahialam in T366765#9961212

Visual examples:

Here is how Special:MyEvents looks now without the wikis:

Here is how the Event List looks now, with no wiki info:

Here is how the Event List will be updated to include WikiProjects (see 'Communities' tab), which is currently on the beta cluster: