With the current ownCloud sharing implementation this is simply not possible. Every shared item appears directly in the "Shared" folder of the user the file/folder is shared with.
Update: At the moment ownCloud (and I guess also nextCloud) allow a user to move around and rename files/folders shared with them. So even if you could enforce a certain structure on your users, they could always change it afterwards.
You could always report a feature request for it (or maybe there even already is one) here: https://github.com/owncloud/core/issues/ .