Some properties, especially the ones likely to be challenged, should usually have a reference. There should be a constraint type for that. (A simple binary should do, we don’t need to define what “nice” references look like.)
This is part of https://www.wikidata.org/wiki/Help_talk:Property_constraints_portal#Improvements_for_2018 . Would start with just binary in the first version and maybe do a second version that looks for real references (and not imported from).