When using backport in a R4 implementation it is possible to represent SubscriptionTopic is as Basic Resource. 
I think that providing a general profile for this resource could be useful to be inserted in the backport artifact. 

At the moment a possible solution for implementers, that we adopted in the IHE DSUBm profile, is to insert the canonical instances of SubscriptionTopic and letting the IG publisher to transform the SubscriptionTopic JSON resource in a Basic resource. (as you can see here the transformation. (see the raw xml and raw json)

Based on the above point, I propose:

• adding a general profile on the Basic resource to define the topic of subscriptions