When I read the proposition of Lionel I thought about the most popular fully featured apps.
For me, requirements to get into this kind of repository should be something like:
UT app (port of existing app to UT, native app)
Popularity the user base should be large enough to draw developers/designers to contribute
Of course popularity is hard to define, but I agree that some advanced apps should benefit from the visibility given by the foundation to help find people to contribute.
I also agree with @dobey that the separation should be clear between community apps, core apps and the OS.
Problems are who could be in charge of the gitlab, how to manage access rights, etc.
I have very little knowledge of how it can be done.
Meanwhile if a developer cannot continue maintaining their app, the forum is a good place to call for a new maintainer and ask for help to keep a nice project alive.