A hyperbolic tree would be very nice especially if colour exceptions can be included.
Also I would like to see an area based heatmap (an overall rectangle made of many smaller rectangles the areas of which correspond to the proportional data value)
A histogram chart in the basic chart engine is overdue.
Functionality wise I would like to be able to set the min & max of an axis via formula/cell value.
There is also room to improve the sparklines perhaps an area chart for example.
Not quite charting, but I also would like to see improvements in the conditional formatting icons.
The pie icons should have icons for 10% increments this could also apply to the bar icons
How about traffic light icons to have additional colours (New, Good, Neutral, Poor, Completed, Missing or 5 grades of performance)