Almost any test fragment assumes that some Jmeter-variables are initialized before execution. When the test fragments are in the main test plan - there is at least one place to describe the required variables: the "Comments" field. But when it loads through the Include controller there is no way to rewiew required variables other then opening another Jmeter window with that fragment. 
For this reason, I suggest the following:
1. The minimal enhancment - to add field in Include controller that is filled with a value from "Comments" of included Test Fragment.
2. The maximal enhancment - to add possibility to mark variables and properties as "required" within the test element with all the ensuing possibilities, allowing to make test fragment as a separate independent "function" with "signature".