Army football seems to be the 16th option only for the Big East. And I'm not sure why. Navy will be joining the Big East in 2015. The Big East could add Army and they'd be able to feature the Army-Navy game as a conference game every year.
Trouble is, Army doesn't want to join. They did not perform well in Conference USA, and are very much enjoying their independence right now. The Army football program is in no condition to compete in the Big East, and even they know it. They need to stabilize the program and have several consistent 6 or 7 win seasons before they might feel comfortable in the Big East. Of course, if Air Force were to join, I think that'd be the deal breaker for Army, and there'd probably be a lot of pressure to join the Big East and unite all three service academies. Without Air Force, however, the Big East doesn't really want Army, and Army doesn't really want them. I don't see the move happening unless the conference truly does take the 16-team approach. But BYU (or someone else?) will have to join in order for that to happen. The Big East would be perfectly content to take Air Force if they wanted to be Team 14 and leave Army alone. That being that, don't expect Army to join the Big East, and if they do, they've probably got one arm tightly twisted behind their backs.