The biggest barrier to playing champ blue line imo is the absence of any aggro "multiplier" skill. I know that the martial champion line is not a priory, but I feel like just a few small changes would make go a long way in making it viable for off tanking.
If I could only make one change to this line, I would reduce the cool down of Champion's Challenge and make it an "aggro copy/multiplier" taunt. Without an aggro multiplier taunt, champions cannot take and maintain aggro from other tanks or high DPS players. This basically rules them out as potential tanks in just about every raid. I would prefer if this skill had a CD of 10-15s (assuming it was an aggro copy skill like what captain's and guard's have), but I could live with 20s (which is effectively an eternity if the tank loses aggro).
If an "aggro copy/multiplier" taunt is just not in the cards, what about a 5s CD to go along with the 5 sec forced taunt? Then at least champs could hold on to one target (although this option would require substantial effort on the part of the champ). I don't mind the existing aggro mechanic, but without the ability to aggro swap in any way, or take control of the situation if the "main tank" falls over, I just can't see a blue champ ever being seriously considered for the more challenging group content.