ConceptMap has been changed so that it can describe relationships between concepts other than equivalence relationships, The list of relationships now allows for related and non-related, and also has some additional equivalence relationships (broader, narrower). 

What is missing - and what I expected to find - is some code somewhere that indicates the type of relationship that is being described - either that they have, or not. I don't believe that it should part of the same code system - it's both orthogonal and not appropriate to try making a required binding. (at best, it could be extensible). 

Most of the use cases for mapping that is not equivalence based involve some kind of property based mapping - concept A has a property that is concept B. Typical use cases include categorization, cost centre, context setting. Given the wide set of capabilities that properties can do in FHIR code systems, it would appear that all non-equivalence concept maps will relate to some kind of property somehow. So I propose that we add to Concept.Group:

property [0..1] 

I propose URL not Coding because we use URL as the master identifier for properties elsewhere.