I would say that one of the main goals of this package must be importing content from old sites using the output JSON format generated by collective.jsonify.
why? collective.jsonify seems to be working in Plone from version 2.1 (at least) to 4.3; so, supporting this format out of the box (while getting rid of Trasnmogrifier on most cases) would be a great step on simplifying migrations.
the import process should take care of migration of standard Archetypes-based content types.
on a new iteration we could be able to allow migration of third party content types by mapping their fields, if needed; or maybe generating those new Dexterity-based content types on the fly.