Add project Bigmenu functionality to core (expandable subtrees via Ajax)

What is the specific case for including this in core? Big Menu is a great contrib solution that can be used on sites with large menus. I think it's safe to say most sites don't have this requirement, the module usage is ~6,500.

Since this works quite well in contrib, and isn't needed by a large majority of sites, it's best kept in contrib. I don't see the use case for adding it to Drupal CMS at this point either.