A polymorphic relationship is dependant of the type of relationship (referenced or embedded) and this bring up situations when you are restricted to defining two different polymorphic classes for the same "behaviour" when working with both kinds of relationships.

This means that polymorphic relationships in mongoid are really: "Embedded polymorphic" and "Referenced polymorphic".

What if it could be possible to do this:

class Business
include Mongoid::Document
has_many :addresses, as: :addressable
end

class Person
include Mongoid::Document
embed_one :address, as: :addressable

class Address
include Mongoid::Document
polymorphic_on :addressable
end

Address could act as a referenced or embedded at runtime depending on context beign called.
I know that it is a big change but it could be interesting if mongoid plans to maintain referenced behaviour in the future