Personally I think the entire game should be redone.
Well not all the game basic stuff like development, housing, demographics should stay.
But the grids should be ridden from the game. Also buildings, subways, rails, etc. should curve.
Various building types should be availble, not all of the world has American/European buildings.
Weather should be incorperated into the game somehow. Snow should fall in the winter, leaves should fall in fall, trees should regrow in spring, etc.
More landmarks should be in.(Grand Central Station, Times Square, Golden Gate Bridge, just to name a few.)
You should be able to choose terrain. You should be able to choose desert,forest, tundra, swamp, whatever.
Help from the advisors should be a little less useless.
This may be just a personal opinion, but you should be able to play already built cities such as New York, Tokyo, Hong Kong, London, Moscow, Chicago, etc.
You've heard the cries Maxis, make it happen!