If you want something challenging, pick a non-humanoid mythical creature, and build a giant one an a lifelike pose. Don't use the duplicator for it, ever, make every block unique and important. If it has wings, give it a curve. Avoid symmetry in the pose and limbs. Make it 100x as tall as a standing player. Make it fully 3D.
If it is indistinguishable from a painting or 3D model at long distances, regardless of the angle it is viewed from, then you have succeeded.
