DataNucleus supports dynamic schema extension (i.e. new persistent classes being added to an existing schema later). Thus it is a prime choice for highly modular applications with some plug-ins providing (abstract) frameworks and others providing implementations. Unfortunately, though, DataNucleus expects at least one implementation being known when an interface is used as field type in a persistent class. Hence, a framework which makes use of interfaces in 1-n- or 1-1-relations currently *must* provide at least one implementation.
It would be a good feature, if DataNucleus did not have this requirement anymore and accepted situations in which no interface implementation is known (yet).