Currently a ComboxBox cell uses the same cell render for the open Cell (the cell in the
list), and the closed Cell (the cell in the button).

There are situations where different rendering would be more appropriate, and
giving the developer an option to change would be good.

See the attached image of a font-size selector :
  it's good user feedback to display the different font-sizes in the list,
  but the different sizes look out-of-place once the list is closed.

See the attached image of the font selector :
  examples of the different fonts look good in the list,
  but using different fonts in the button won't always fit,
  and can make the button look out-of-place.