Bricklink MOC Contest
They've got all the rules, but to sum up.
Buy a Lego Creator set from 2014/15 with 400 pieces or less and make an alternate.
Bricklink's List of acceptable models Pay attention to the year, see if you've got what it takes.
They're offering a cash prize if you need an incentive...
Of course, I just received Emeral Express for my Birthday...so I'm going to go small and be as dastardly as 31015 allows...good luck
You can scan over several of the sets here; remember the 400 piece maximum limit.