MDL-27908 was introduced the feature of adding items in a given point in the navigation tree. Surely that will lead to modules (for example), having all their options together (top). See MDL-27908 for example.
I think it could be interesting to be able to add one separator item, in a very similar way to the current add_node() implementation, i.e. specifying where.
Those separators really help visually to group options IMO.
For your consideration, ciao