Advertisement Red-Eyes Black Dragon is another super cool card, says Wu
This is a decent way of just trying it
You can find more about them in out Pack Guide
Gengar Exs main drawback is the weak stats of its pre-evolutions, Gastly and Haunter, making it a mountain to climb to reach Gengar Ex
Id be happy to run a few copies of Sphinxs Revelation alongside Obzedat, but such a greedy combination of UU, WW, and BB is only achievable in a heavy Esper Control list, which I wouldnt build until the new aggressive decks appear and stabilizeand which will probably not run any Orzhov cards besides Obzedat
Alternatively, they could keep their Earth attribute while getting an erratum that specifies they are also Metal or Wood, so as not to change their effects if they involve Earth attribute monsters in some way