The idea of having Phabricator projects for Wikimedia sister projects has been suggested several times for several reasons, and it starts to be the time to discuss it properly.
From a #Phabricator team point of view the answer is simple: if Wikimedia sister projects want to have Phabricator projects, they can have them. It's just that there is hundreds of them, and they have a potential of opening cans and boxes with unexpected content inside. :) For instance, are we ready to receive hundreds of tasks in dozens of languages and a request to localize Phabricator? Or to have users starting VillagePump-like discussions when the admins of those projects are not even following here? We need some coordination before approving the first project.
As usual, a good first step is to define needs and specific usecases.
* **Better organization for some specific projects**. mediawiki.org, Meta, Commons, wikimediafoundation.org have very specific needs and well organized communities used to work in English first. We might just want to open the door to them right away.
* **A bridge between non-technical users and developers**. Not many users will know that a problem is related with the Proofread extension, but most of them will be able to tell that their problem is happening in #Wikisource. The #Wikisource project can be watched by tech ambassadors and other users knowing well that community and its technical links, people that are in a good position to help those users and triage those tasks.
* **Localization step 1**. As of today, Bugzilla and etc are English-only tools, but only the simple fact of having editable descriptions provides an opportunity to be more flexible. What if a user would be able to file a task in French for #fr.wikisource.org (and/or #French #Wikisource, another detail to be defined)? Then the contributors watching these projects could help triaging the task in French for that user, and translate to English if there is a valid new bug.
Feel free to add more.
PS: marking this task as Low priority only to reflect that the ongoing migrations have a higher priority. The own editors of sister projects will probably understand better this discussions and what they want to do with Phabricator once they see Bugzilla and a critical mass of Trello / Mingle projects migrated here.