A similar idea had cross my mind some time ago. But i thought about it not as a challenge but more as a helping hand to Corona newcomers. The idea is, let's say someone puts fully modelled scene without materials, textures and lighting to be downloaded and then everyone who wants can download it, apply own materials, adjust lighting and render settings. Then render it and upload image and scene to be availlable to everyone. That way, when new guy comes to forum, he can look at such topic, pick scene that he finds most attractive, download it and study how it set up.
That would be really useful "challenge".