It would probably the greatest cross-over story ever told if this would happen, and fanboys and fangirls will squeal until their lungs pop out. Although there was one guy who managed to create a fan fiction pitting against Commander Shepard and Sephiroth (from Final Fantasy VII) in an intergalactic battle.
You can check the fan fiction from this link, but unfortunately the fanfic wasn’t updated for quite some time now.
But of course fans will still crave for more, and they want it from the creators themselves. And luckily Square Enix is releasing a new DLC for Final Fantasy XIII-2. The DLC will feature a new costume pack for Noel and Serah that features the armor that Commander Shepard used in the Mass Effect trilogy. Altough it may only be a costume (dubbed as the N7 armor), you can imagine that the duo were part of the N7 program to save the galaxy.
Also included in the DLC is a new coliseum battle which features infamous bosses such as Ultros and Typhon from Final Fantasy VI. Defeating them will give you a chance to get their crystal and they will join your party.
The DLC is scheduled to release in Japan on March 27 for 300 Yen (PS3) and 240 Microsoft Points (Xbox 360), this is also expected to release in the US but no release date was announced.