Question about datatype maps

The OWL specification has a datatype map as a parameter. It's unclear  
to me what are allowable extensions in the realm of datatypes, and  
whether such extensions would be desirable from an interoperability  
point of view. Now that we have a wide range of datatypes, do we need  
to support extensibility here? Our experience OWL 1, OWL 2, that  
choice and semantics of datatypes are not a slam-dunk obvious choice,  
raising questions about whether sanctioned extensions to OWL in this  
dimension would be beneficial or cause more trouble than they are  
worth. Of course nothing would prevent unsanctioned extensions - my  
question here is of what we should encourage.

-Alan