CMH-DFW with Comair or ASA would be nice, but I think the demand for the market might be satisfied already, since both American and America West fly that route, with a combined total of 8 daily flights. The same goes for Boston, with America West Express and American Eagle both flying a combined 9 daily on the route. However, the BOS flights mostly fullfil O&D demand, so if Delta were to provide some good connection opportunities, I could see ACJ flying 2 or 3 daily nonstops between CMH and BOS.
Also, Columbus' population is now roughly 1.54 million, and is the fastest growing metropolitan area in Ohio.
Delta Express did a seasonal CMH-FLL nonstop 2 years ago, but never brought it back. With the Miami/Ft. Lauderdale area averaging over 500 passengers per day from Columbus, I've always found it hard to see why no airline can seem to maintain a nonstop flight to the area.