I was thinking: the 4 elements have the starting monsters, and Holy has Chrono. All monsters that everyone can obtain, and with enough grinding, they actually become pretty good. They are also all related to each other and to the Statue Chamber event because of their UEs.
The closes thing shadow has to that is Bitterbeast I think, which is ok but misses the connection to the rest. Because of that I personally would like so see a new Shadow monster that, similarly to Chrono, everyone can has and evolve to legendary. I wouldn’t know about his design though.