The idea is to split the app-schema module in two:
The first part would contain everything that helps with complex features in general: building them, evaluating filters against them (property accessor, x-path evaluation), building complex feature types from XSD. These classes do not rely on specific schemes like GML, etc... apart form XS.
The second part will contain anything that is app-schema specific: creating complex feature datastores with mapping files.
The second part would continue as the gt-app-schema module. The first part would split off and become a new module 'gt-complex.
gt-complex should get its own page in the GeoTools User Documentation.