For the "middle set twist" of Scars of Mirrodin block during Mirrodin Besieged, the Mirran resistance members tried to overwhelm the Phyrexians with all-out unified attacks, represented by the keyword battle cry. The mechanic was eminently simple and very well-received, especially for the tournament-quality mythic rare cards it was on, but it failed to reappear after that set.
However, when popcornbunni asked Mark Rosewater on his Tumblr blog, "Is there any chance Battlecry could make it into the core sets? Or is it a Mirrodin-specific rally call?" Rosewater replied:
It’s the kind of mechanic we could do in a core set.
Core set returning keywords tend to be weighted towards a number of keywords if not exclusive to them outright, and are often combat-related. Magic 2011's scry was exclusive to blue, black, and artifacts; 2012's bloodthirst was in black, red, and green; 2013's exalted was white and black, with one land also with the mechanic; and 2014's Slivers were primarily white, red, and green with one each in blue and black.
This means that although blue has so far gotten the short end of the stick, and battle cry does not necessarily fit in that color, white and red alone still has yet to be represented, and that is, in fact, battle cry's natural color combination.
Regardless, given the saturation of white over the past few years, battle cry is likely not in Magic 2015; this may be the time for R&D to introduce its vaunted "blue combat mechanic."