Are we at the point where we need a sub forum of each main distro of Linux?
I think the Linux & Unix category is at a point where it could justify having subcategories of some kind, but I wouldn't say it needs to. There aren't that many users in the Linux & Unix section. Nore is there a mass of daily posts.
That is, one of Debian based, one for Red Hat, Slackware, &c.
Most forums go with an OS specific approach, not an origin approach. I'm neutral on the matter, as I see the merits of both approaches.
Some users may not know the origin parent of the distro they are using, and therefore post in Linux & Unix, rather than the appropriate subcategory. This could perhaps be solved to some degree by listing common derivatives in the category description. As for the posts that still end up in Linux & Unix, I'd say leave them there. It's not the end of the world, and that way it doesn't make a ton of work for moderators.
What happens when a specific derivative starts to take over a category (eg: If our Debian section became mostly about Ubuntu)? Will we solve this with distro specific subcategories within? If so, will this be applied recursively (eg: If the Ubuntu section within the Debian section became mostly about Linux Mint), or just stop there? If applied recursively, there could be a problem with posts for OSNAME in an upper category despite there being a category for OSNAME further down, because users may not know it's there. I'm not sure how this could be reduced.
How will subcategories affect the tutorials category?
Leave the main forum for generic questions and discussion?
I would say generic posts, and posts that don't fit into a subcategory. I also think that posts which fit into multiple subcategories should be welcome in the main if the OP wants it there.
If an origin parent approach were implemented (as suggested) then I think only Debian would presently be able to justify a category, as most posts seem to be about Linux Mint family members, and Ubuntu family members. Alternatively, if an OS specific approach were implemented I think only Linux Mint would presently be able to justify a category. Regardless of approach, I would be okay with having subcategories
so long as creation of each subcategory is justifiable.