Users rely in notifications to support a wide range of daily activities. If we do a major change like introducing cross-wiki notifications, we may not want to expose the changes to everyone at the same time. Exposing changes to a more limited scope will be beneficial to get feedback and discover issues at a pace we can respond to.
Some options (feel free to suggest more):
- **A beta feature.** Beta features allow users to voluntarily opt-in (and opt-out when they have problems), presents the feature as an early development open for input and welcomes and facilitates feedback. Currently beta-features need to be enabled on a per-wiki basis, so we may need to work around this limitation due to the context of the project. For example, allowing global enablement (the feature is marked as global and when users enable/disable it, affects all wikis), or some kind of propagation (when a cross-wiki notification is opened, the beta feature is automatically enabled on that wiki).
- **Site-specific availability.** Cross-wiki notifications can be initially enabled on a couple of sites, and notifications will be visible across those two sites. All users participating in just those two wikis will have the cross-wiki experience (feedback limited by the volume of users). Users not participating on those users won't be affected. Users participating on those sites but also others, would get a mixed experience that may be sub-optimal (getting some notifications but not others without apparent reason). The selection of the sites can alleviate the issues: it is easier to understand that "notifications from Commons are now are visible in Dutch Wikipedia" than "notifications from German Wikipedia are visible in Dutch Wikipedia". The later seems more of an arbitrary language decision.
- **Notifiction-specific availability.** Cross-wiki notifications can be initially enabled for a specific kind of notifications (e.g., talk page notifications). Users will access a specific kind of notification regardless of their location. Since most users get different kinds of notifications, everybody will get a mixed experience.
- A combination of the above.