In the North America Challenge, I served almost 100 destinations from SJU with E120's and A319/A320 equipment. Remember, frequency wins so you can, indeed, serve MCO, MIA, FLL, TPA, and JFK with 737's, you just have to fly the route 20 times a day. That is admittedly a bit more difficult with the new minimum departure intervals, but it is doable. Then you can add (or continue flying, in this case) 767's to serve Europe and still keep your fleet at 3 types and your costs at a minimum. Now, I used 32X, but there were several destinations I could not reach with them, such as PDX, SEA, and YVR, that is why I would recommend 737 NG's as your final narrow body fleet type for SJU.