Tony,
Since 18 mm on a 1.6x sensor is often not wide enough, Canon developed the 10-22 to be very similar to the 16-35 on a 35 mm body. As you already own the 100-400, a good choice would be to add the 10-22 and the 24-105 IS. Of course the 17-85 is the other option, but 17 mm is not much...