I've dreamed up a bunch more prompts today. Remember, dark themes don't necessarily mean gore and murder and sadism, though all that's good, too. ;) What it can mean is psychological darkness and irony and all kinds of interesting stuff.
It's multi-fandom but there are plenty of DC prompts over there. And the cool thing is when the claiming starts on February 4th, each prompt can be claimed multiple times so it isn't first come, first served. Everyone gets a chance! :)
You can also prompt anonymously. Have fun! :)