I was feeling a little left out just reading this thread, so I decided to join in
One awesome card that I wish had an extra color is absolutely
Atarka, World Render. Adding black would open up possibilities for awesome removal, more cool dragons, and a whole bunch of tutors. And really, could anyone resist the juicy double-strikeness that would come from
Boltwing Marauder and Atarka's effects? That's a lot of damage.
Second thought I had was adding a colour to a very commonly played commander, but most that I can think of really don't need it. They're great the way they are. However, making
Emry, Lurker of the Loch Azorius, Dimir, or Esper would open up a lot more artifact combo cards (like
Time Sieve) and all those classic commander staples in those colours.