Opposites - since it seems to be the form to suggest new or rekindled ideas so I'll do that too :) How about themes like colour: red , blue , yellow , etc. the challenge would be to find a shot that was interesting containing that colour, no good shooting grass for green, of course you could if you can think of an original way to make the shot interesting. Have to have an agreement not to manipulate the colours I suppose, no blue daffodils. Thought number 2: Everyone has the same image to digitally manipulate in anyway they choose. Part of the entry would be to say what you had done so that others could learn.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html