Previously we have had discussions about having a childrenType set on the document, which forces all top-level blocks to be a certain type.

In the past I thought this was a good idea, but now that I have considered the UX, it seems pretty bad. Because, in the example where the user has a list, the users may want to put things above the list or below the list.

So I think we should keep our current data model- and help the user create lists by auto-grouping a new "top-level list" under an empty block.