Currently OneNote have a 5 layer deep possible hierarchy for a given Notebook:
- section group.
- section.
- page.
- subpage (with collapsible option).
- sub-subpage (without collapsible option).
I cannot see why not to add infinite nested pages. It will give users the full control of their organization of notes.
Another option is to add a type of page (say "hierarchy page") which allowed to have infinite nesting of pages.