 One area that philosophers might profit from is in the investigation of the relational observables that appear to be necessitated by diffeomorphism invariance. For example, since symmetries (such as the gauge symmetries associated with the constraints) come with quite a lot of metaphysical baggage attached (as philosophers of physics know from the  hole argument),  such a move involves philosophically weighty assumptions. For example, the presence of symmetries in a theory would appear to allow for more possibilities than one without, so eradicating the symmetries (by solving the constraints and going to the reduced, physical phase space) means eradicating a chunk of possibility space: in particular, one is eradicating states that are deemed to be physically equivalent, despite having some formal differences in terms of representaton. Hence, imposing the constraints involves some serious modal assumptions. Belot and Earman (2001) have argued that since the traditional positions on the ontology of spacetime (relationalism and substantivalism) involve a commitment to a certain way of counting possibilities, the decision to eliminate symmetries can have serious implications for the ontology one can then adopt. Further, if some particular method (out of retaining or eliminating symmetries) were shown to be successful in the quest for quantizing gravity, then, they argue, one could have good scientific reasons for favouring one of substantivalism or relationalism. (See Belot, 2011a, for more on this argument; Rickles, 2008c, explicitly argues against the idea that possibility spaces have any relevance for spacetime ontology.)