I'm not completely sure I understand the idea: do you mean to say you would like to add arbitrary grouping within in an item in order to be able to hide/show such groups (like photo selections can be opened and closed)? In that case, I think this isn't something we've considered.
On the other hand, we'll be looking to make the panels and item view work with multiple items. Right now, if you select multiple items, you can use the metadata panel for batch editing, but the photo and note panels have no special functionality; plus, if you switch to the item view you will, by definition, see only a single item. We have been exploring ideas how to add 'batch' features to the item view, allowing you to open multiple items at the same time. If something like that were possible, I think you would be able to organize your items a little differently (e.g., each item is a page) and group those items together (right now, you can 'group' items via lists or tags, but we're considering more advanced ways to group items as well). If the item view supported showing multiple items, it would very natural to add a way to open/close an item's photos similarly to how it's currently possible for a photo's selections.
That said, we've also been thinking about how to improve the organization of photos within an item (e.g., allow different ways of sorting etc.) but as far as introducing further ways of grouping I think it's more realistic that this will happen at the item-, not the photo-level.