Currently Flarum has the following default hierarchy of groups when setting permissions, each being broader than the previous one:
Admins -> Mods -> All Users -> Public
This is good but there's a particular issue when you need to limit some functionality and allow it only to trusted/established users. For instance, you may want to forbid private messaging and file uploads to newly registered users.
Currently there's no easy way: you have to create two separate groups, one called "Newbies", one called "Regulars", then apply separate permissions for both groups, then use Auto-Moderator to move the users between the two groups. And the AutoModerator extension is actually not working as of late, according to some users. Besides, it's just too abstract and over-complicated.
I propose that Flarum Core includes a new group in the hierarchy:
Admins -> Mods -> Trusted Users -> All Users -> Public
The "Trusted Users" will be those who have enough posts and have registered no less than a certain amount of time. Those will be configurable.
I believe that should be in the core and not an extension, since it fits nicely into the existing default group notion, where there's hierarchy of trust. When you, for instance, give permission to "Trusted Users", then newbies will be excluded. When you give permission to "All Users", then both trusted and newbies are included.
No need for artificial groups and moving between groups and using complicated extensions.
As an added bonus, this will make it much easier to fight spam. The First Post Approval extension won't even be needed, its functionality can be directly enabled in Flarum by allowing only trusted-users to post without an approval, etc.