If other apps should be included the Software Policy needs an addition moduleset. Maybe it’s worth considering what a future set of core apps could look like and based on this try to estimate if there would be enough resources to maintain the required app quality for more than the core apps.
I noticed that the linked wiki page lists https://wiki.gnome.org/ReleasePlanning/ModuleRequirements as requirement which in turn states “This page is totally obsolete and exists for historical interest only.” Maybe this is not a good requirement 
Some brainstorming for less-technical requirements if it helps
- The app provides at least what is the minimum that average users would expect from the apps name (a bit more than minimum viable product)
- There is some minimal user testing that ensures that there is no uniform obstacle in using the app for the first time.
- Basic design review from design team
- Basic code quality review from ???
(I guess I’m a bit on the conservative side concerning the quality of new GNOME apps.)