Limbo

It would be cool if the background image was a bit bigger than the level and shifted left right up and down slightly so that the clouds appeared to shift (based on user movement/input).

Consider adding a lose condition to spice things up. Like an "archangel" that will snuff you out if you touch him (and maybe once all the other angels are converted you can take him out too!) And if you want to make it even more challenging you can increase the archangel's speed for each angel that is converted.