The May 2013 Issue of Automobile Magazine is out and the publication contains some interesting new speculation about the 2015 Mustang. Some of this information has been known and/or widely speculated while other info is new.
The 2015 Mustang will premiere at the New York Auto Show on April 16 or 17, 2014. It will be slightly smaller and lighter and come close to retaining the current car's muscularity. The current 5.8L V8 in the GT500 will not fit in the slimmer 2015 Mustang. It will have an independent rear suspension (IRS).
The article also suggests that 6 (yes, six) different engine options will be available for the S550:
EcoBoost 5.0 Coyote Engine for the GT500
N/A 5.0 Coyote Engine for the Boss 302 (and presumably the GT)
EcoBoost 3.5L V6
N/A 3.7L V6
EcoBoost 2.0L 4-cylinder
Looks like the V6 EcoBoost idea isn't totally dead, which is what I was hoping would show up in the new Mustang. I don't know where they're getting "6 different engines" though, I only see 5 listed.