Is it possible to have a sub-functional area to be shown in the Content Panel?
No, there is no way we can have sub functions within the content panel.
I don't think if there is anything like that coming up in near future. However you can post your idea on idea exchange forum and a use case about it.
Anaplan has categorization feature which basically categorizes the functional areas of the modules but it doesn't get reflected to the content panel. Atleast that's what I have seen so far.
I have posted an idea about this and let's see if that's taken up.
There is not option for a sub Functional Area for now in Anaplan.
If this is related to the end user viewing, I would suggest to consider Categories in the New UX. You can align your pages into categories in the New UX. But again there is no option of cascading more than one categories there.
The best way I achieve this in any of the classic contents/new UX is using numbers and then categorising based on the numbers.
e.g. Demand Planning has 2 sub categories - sub categories as 1.1 and 1.2 with respective modules and dashboards per the role. Remember if you have no modules/dashboards per the role, this will not be visible. So in case you create 1. Demand Planning with no modules/dashboards, it will not be visible.
As @Misbah suggested, you can always post an idea in the IDEA EXCHANGE which can be considered by the product team per the priority.