There's not really a "one-size-fits-all" solution for folder structure; it really depends on how you're using the documents and what makes sense for your organization/processes.

We have 3 main repositories, and they all use drastically different structures based on their purpose.

For example, one repository is broken down by department, and each department folder has subfolders based on subgroups/teams or processes.

Another repository is more document/archive focused so the folders are organized by document type with subfolders that divide them into subgroups based on prefixes to avoid having too many items in a single folder.

In general, I'd say to think about security first; folder-based access control is by far the easiest and most efficient way to manage content permissions so that should be your first consideration.

For example, if you're building out a folder structure for HR, you could set it up so everyone in HR gets access to that top-level folder (this entry only), then you could set up subfolders so different members of HR get access to different subfolders depending on their role.

The next thing to consider is how the documents are used. For example, archival storage can be handled much differently than folders for things that are accessed/updated more frequently.

It's all about finding a good balance between effective access control and ease-of-use, both of which depend on or your organization's specific needs and use cases.