Ooh! What about a ghost dog? It'd be a light bluish-gray with a little bit of an ethereal glow around it, and it'd have glowing red eyes! That'd be so cool...
Staying on the subject of Halloween (cause I LOVE it and it's not that far away!), I'd LOVE to see something like an all black dog with just a small white skull and crossbones on it's hindquarters. Some other October-realted ideas are...
Jack-O-Latern dog: A bright orange dog with floppy ears. It'd have glowing yellow eyes and a little yellow traingle over each eye. It's nose would be yellow and it'd have a yellow stripe around it's mouth. For some reason, this is just saying 'PPS' to me, so maybe you could have a full grown and a PPS version.
Spider dog: A pitch black wolf with prick ears. He'd have bright red eyes and the hourglass mark of a black widow on his back. Simple, but cool

Gravestone dog: A wolf with prick ears that was colored to look like a statue. On his hip, shoulder, or side the letters 'R.I.P.' would be etched out. This one's probably my favorite
