Owning up to my nick, I'd say do a Hellblazer show. I mean, if one wants to explore the magical side of the DCU John Constantine would be the way to go... though not the Keanu Reeves version. It could be done like a DCU Supernatural with guest stars like Zatanna and so on and so forth.
Hellblazer would be a cool idea. However, as a WB production and (almost certainly) CW show, the main downside is that the setting will have to be a US-based city rather than England. Also, rather than Supernatural, I think the main source of inspiration would be Buffy and Angel instead.
Another idea, as Spider-Fan83 touched on, is Martian Manhunter. It would work as a perfect triumvirate with Arrow and the proposed Wonder Woman series; dividing the DC television universe into separate 'realistic', fantasy and sci-fi settings.
My pitch: MM's powers are limited to telepathy and shapeshifting (cost-saving and makes him less god-like). He is sent to Earth in order to covertly capture/eliminate Martian criminals who have infiltrated Earth in a plan to invade/conquer it and to cover up the existence of aliens from humans. Later seasons would eventually lead to the existence of Martians getting out, leading to (or as a result of) the arrival of other alien characters; allowing the show to take on full on space opera elements.
Alternatively, a more classic idea: MM is accidentally teleported through space and time to Earth as a result of a human science experiment gone wrong. His arrival results in the destruction of the lab in which he is the sole survivor. He later learns that he's travelled into the future, and that Mars is now a dead, barren planet; and his people wiped out. The story then revolves around MM coming to terms that he is the 'last Martian' with no hope of ever returning, hiding his existence from the government, and doing the one thing he knows - protecting the innocent.
The problem with the second though, is that it would incorporate fewer sci-fi elements than the more radical take on the Martian Manhunter.
