You see, JetBlue's Terminal 5 doesn't have customs and immigration areas, so it can only depart international flights. It cannot now accept international arrivals, which instead go over to Terminal 4 and endure those long queues. We know from many past experiences. T5i means those JetBlue planes will come home to their own terminal, to their own customs and immigration, and also make transfers to other domestic JetBlue flights far smoother. There's even the news that T5i will be able to handle wide-body airliners operated by JetBlue's partner airlines, though we doubt you'll see an Emirates A380 rolling up in there.
While there aren't any renderings to share yet, we do know that T5i will be 145,000 square feet and completed in 2015. The ground breaking is this summer, so if you're flying out of Terminal 5 anytime soon, watch for some guys digging ribbon-tied shovels into the dirt next door.