i've checked out quite a few wallpaper managers - most have the ability to change the wallpaper at timed intervals, say, once every hour or so.
how about having a wallpaper manager that changed what was on display at a specific time set by the user?
this way you could set up your morning desktop wallpaper that you like to be greeted with, this could then change at midday (or whatever time you specified) and it could then change again in the evening (and again after that towards dusk).
obviously i'm trying to describe something that mimics the changing pattern of daylight outside your window - you would just have something similar presented to you on your monitor. you know, sunrise, sunset, starry night, etc.
taking things further you could have wallpaper set with different times displayed on it - the wallpaper then changes on the hour every hour (or whatever interval you have set that corresponds to the time prints on you wallpaper set).
if this has already been done elsewhere, please let me know, i've hunted around and can't find this specific feature.
i'm no programmer but i would think this a simple thing to code