Since the 1990’s several Chondrichthyes have been proven and triturated, allowing us a reasonable picture of this group. However, several large gaps in out knowledge of the 
Chondrichtyes still exist.
The greatest gaps in the extant Chondrichthytes is the Holocephali. Currently no members of this family have been either potentised or proven. In the Elasmobranchii, the Squalomorphii, have not been potentised or proven. Of these the dogfish, or Squalidae, would be ideal candidates for proving, due to their close relationship with humans, and the frequent encounter of their egg sacs, called mermaids purses, in tidal ponds.
In terms of fossils, the Eugeneodontiformes, Symorida, Xenacanthiformes and Hybodontiformes are all completely unproven and unpotentised, and would all make excellent candidates for proving.
Image Credit: https://en.wikipedia.org/wiki/File:White_shark.jpg
