If you look back through past competition titles, I'm sure you'll find quite a few possibilities.
In addition it could be possible to pick a particular technique, Macro, Nightime, still life, infrared...
It depends somewhat on how wide a range of techniques you want to include - something such as RED would give the option of trying loads of different things. Do you want it to be a Jack of all trades approach or more a master of one.
I prefer JOT myself
