My first thought would be to see if you could avoid heel strike with the existing bag. What works for me is to mount a bag transversely on the rack, and then attach the bungies not to the "ears" where they are usually attached, but to the front of the rack itself. I usually put both bungie hooks in the center "box"* at the front of the rack; there's just room for both hooks. At least for me, this keeps the bag back far enough that I don't have heelstrike issues...this works well (for me) even with a folded B-Bag, which is not a small bag.

*If you look at the rack from the top, it consists of four longitudinal metal ribs. At the front of the rack, the two center ribs continue past the edge of the rack and form a 1" x 1" downward sloping "box" - I think it is there to protect the rear brakes. Anyway, this is where I attach the bungies.

I have a couple of carradice saddlebags that I sometimes use with the B. They attach to the saddle so you shouldn't have heel strike issues at all. They also have the advantage that you can park or even fold the bike without having to take them off. They have the disadvantage that it takes a minute or so to take them off the bike and aren't that easy to carry.

Here's a picture from Carradice's website:

On a B, the bags are well above the rear rack.