That's a good one with the black dogs. As a matter of fact in the origall Omen movie they have rotwierls. Very creepy.Is it an inside or outside haunt..or both.
Here's just a few I've thought of "off the bat" that seem quite do-able...
Cannibal Hillbilly feast. (A la texas chainsaw/Hills have eyes).
Psycho type guy with his mother's head in a box or mothers corpse in the chair.
The family secret..guy keeps his deformed, deranged twin brother/sister in the basement/attic/locked room. (Could do a take on the MIB prop effect for this).
In England, there are many local legends of Ghostly black dogs with glowing eyes that are supposed to bring death or bad fortune to any that see them...And what about "the little people"..faeries and leprechauns and such. Are these classed as "urban legends"? It would be quite easy to do a horrific take on "The wee folk". This year I'm planning on a particularly nasty version of The Tooth Fairy to grace my garden.
Just a few suggestions, I'll post some more if I can think of any that may be practical to do from a time and cost perspective.