It would be neat if production buildings could upgrade to match the aesthetics of the areas they are placed in, lots of the lower tier buildings look out of place in later game cities- the tavern specifically comes to mind (wasn't there a tavern splendor package prior to 1.0???).
As I said in another post- I'd love to see more building expansion options in general, the feature is fantastic. But an aesthetic upgrade would be cool too, even if it doesn't change production stats at all.