America_West: I believe that some of the routes you mentioned could work. Flights between Columbus and London, as well as Frankfurt, have been a goal for the Columbus Metropolitan Airport Authority for several years. The closest Columbus has ever seen to nonstop service to London was a short-lived codeshare between America West and British Airways, where CMH passengers would travel to PHL on America West, and then connect onto a British Airways flight to London. I believe America West and British Airways still have a codeshare partnership, so if America West were to expand their Columbus hub to a point where a sufficient amount of connecting passengers would be traveling across the Atlantic, I could see daily service to London with British Airways, as the combined O&D demand in addition to connecting traffic would most likely be able to support the route. Though surprisingly enough, the most popular international destination from CMH is Tokyo, Japan. The main reason is because many Honda executive in Marysville travel to Japan from CMH throughout the year.
Flights between Columbus and Mexico City would be nice, however as with London, more connecting passengers would probably be needed to make the route profitable. Columbus' Mexican population is growing, and I'm under the impression that there is a healthy amount of business conducted between Ohio's capital and Mexico's, however it probably isn't enough demand to warrant nonstop flights. Both Delta's hub in Cincinnati and Continental's hub in Cleveland, which are both much larger than America West's hub in Columbus, don't even have nonstop service to MEX.
Service to the Miami/Ft. Lauderdale area is a definite necessity from CMH. With the two cities combined, over 500 people fly daily between Columbus and Southwestern Florida. Miami is also the country's top airport for flights to Latin America. Governor Taft has strongly encouraged increased trade between Ohio and South America, and nonstop service to the Miami/Ft. Lauderdale area would help immensely. If an airline were to start flights to MIA, FLL, or both, it more than likely would not be with jetBlue. As of now, their main focal point is feeding passenger traffic into their JFK hub. I could only see jetBlue flying nonstop CMH-FLL flights if they were to start a Delta Express-type operation in Florida, where flights would mostly connect the Midwest to Florida. To Ft. Lauderdale, service would probably be flown by either Southwest, Delta Express, or America West. If Miami service were started, it would probably be with either American Airlines or America West.
I think CMH-JFK service with jetBlue isn't a matter of if it will happen, but when. Columbus is on the airline's list of possible cities, and Columbus currently does not have nonstop flights to JFK. Currently, however, there isn't any room at CMH for a new airline. Once the Concourse C expansion is completed, seeing jetBlue's A320's in Columbus is definitely possible.
I too would like to see some more tropical seasonal destinations from CMH. A definite way to get more flights between Columbus and the Carribean would be with charter operators. Currently, CMH's charter traffic is severely lacking, with only one Sunday nonstop to Cancun. I would like to see charter traffic to Punta Cana and Puerto Vallarta.