Even before the resurgence of the Eldrazi, Zendikar was a dangerous place. Hence R&D introduced the first creature type with mechanical relevance in years: Allies, which functioned like a secondary class type, all of which cared about other Allies entering the battlefield. This "like-Slivers-but-not" model proved very popular, and there were many and diverse enough Ally cards to add variety to Limited, so they became a defining part of Zendikar block for many people.
One such player asked Mark Rosewater about Allies on his Tumblr blog and received a favorable response; when 26toomany asked "I really enjoyed Zendikar block -- the large-expansion sandwich, the full-art lands, and, most importantly, the Ally creature subtype. Will we see any more Allies soon?" Mark replied:
I strongly doubt we’ve seen the last of Allies.
Ally decks also did rather well in Constructed; another block with Allies, possibly the vaunted "Return to Zendikar," would probably boost decks of that type in Modern a great deal.