"Kird Apes"
Another key invention of Doug Beyer's M13 design team was the cycle of uncommon creatures that powered up off of different land types, most notably Flinthoof Boar. We called them the "Kird Ape" cycle internally, as a nod to the famous Arabian Nights creature they were based on, and for M15 we decided to bring them back, going the other way around the wheel (so the red one powers up off of Forests, etc.). These powerhouses make for great rewards in Limited and reward "splashing" in very satisfying ways.
Color Lords
As if ten different two-color deck-building themes weren't enough, I also wanted rewards for trying to play a one-colored deck. To that end, we made an uncommon cycle called the "Paragons" that tempt you to do just that! The work Mark Globus did on M14 design highlighted for me the power of simple-to-achieve missions in deck building, and these lords do an admirable job of setting you on such a mission. Whether you're drafting or playing casual Constructed, the Paragons are sure to give your monocolored decks a serious shot in the arm!
Slivers!
What? Weren't Slivers M14's thing? Yes, they were, but I didn't want people who made Sliver decks in M14 to never get any new toys to play with. Because Slivers don't reside on Theros, M15 was the only other chance we had to get new Slivers into Standard alongside Galerider Sliver, Predatory Sliver, and the rest of the M14 crew. There is a cycle of five uncommons, many of which play well on their own and are devastating in combination, as well as a new mythic rare Sliver that every Sliver aficionado has been waiting for.
http://magic.wizards.com/en/articles/archive/magic-2015—its-there-2014-06-23