this idea applies to both Microsoft Dynamics 365 for Marketing and Microsoft Dynamics 365 CE since it seems the constraints remains the same in both products:
It would be useful to be able to use custom entities as a source for segments in D365 for Marketing or as source for Marketing lists in D365 CE.
Currently it seems not to be the case and it has been confirmed by several experts as not possible to extend, which limits the sources to a set of existing native entities ( Leads, Contacts, or Accounts).

Not being able to extend it is a pain as some customers extend the data model with entities that should be used also for marketing purposes, and they to not understand why they are stuck by product capacities while they can customize it easily otherwise.
Category: Email Marketing