Change in visuals: great idea
For those who say it's unnecessary to change the visual aspect of buildings at the max level: well, it's 'unnecessary' to change the buildings at all from 1 level to another!
The fact is that the change of visual appearance is actually a vital part of the game's success. If nothing changed, ppl would become very quickly bored--especially since everything takes so long in the first place.
This idea ought to be immediately implemented.