Before releasing a new theme, I'd really like to see an updated Rural, Alpine, and Desert (think Primm Valley or Painted Desert)--adjust colors, add new foliage, etc.. get them up to snuff with the newer themes. Maybe throw Links in there too, but with Highlands now, not so much of a priority.
As for entirely new themes..
Urban would be nice, but I think they could achieve that with just a couple new universal backdrops.
Global assets are all that we would need to have a "Custom" theme. It would also ensure that the current themes not become obsolete.