As I said I've been thinking about creating a kind of short cut InputfieldType similar to ryan's idea of repeatable fields. But this one would allow to manage a page's hidden child pages of a set of allowed templates skipping the way through the page tree. It's something I think would help people editing contents to work faster and avoid mistakes. And it still would keep the data hierarchically consistent within the tree.
In the case of Repeatable Fields I guess it's all based upon FieldtypeMultiple and ryan stores the IDs of the hidden pages representing the single repeatable elements in the DB like in FieldtypePage.
The point is, that - in case of my Fieldtype's idea - it wouldn't make sense to store any additional information in the db, as all information would be already stored within the pages' relations within the page tree (children, IDs, sort values). But when I'm not totally wrong, the Fieldtypes expect a DB scheme to be provided for installation, saving and getting values. Which methods do I have to override to work around using an own DB scheme and just provide data from API calls? Is it possible at all?