Assuming that the next generation would all be LTE (there's plenty of room in the 9x00's for more devices), I'm thinking Bx00's or go down to 4x00"
So a B100 would be an LTE Pearl, a B300 Curve, B500 could Possibly be the LTE Storm, but the S3 is starting to look like it's an LTE and it most likely has 9580 as it's number.
heck, you could drop a zero and make it the B10, and the B11,B12 for different versions (T9, eta)
I say they get away from numbers and come up with cool names. Not like bold pearl storm etc, but like the blackberry invincible, or or blackberry triton.