I looked up the largest cities, and here are the ones without a UA, AA or DL hub: SAT, SAN, SJC, AUS, JAX, CMH, IND, SEA, BOS, ELP, BNA and MEM. Pick one.
I think the more interesting question is CLE. Is the mini-hub model a good one? If so, promoting BOS, JAX or maybe PDX to mini-hub status might be the thing to do.