Although R&D head Mark Rosewater may have a strict policy of not being able to see unsolicited card designs, for legal reasons, he's perfectly happy to take suggestions about literally anything else in Magic. And one thing people always ask him for is a particular block setting to be used in the future - most popularly "return to a past plane," but running a close second is "plane based on a real-world culture," like Kamigawa is for Japan or Theros is for Greece. But one Tumblr user had an idea, along the same lines but much wider in scope, that impressed Rosewater.
When mcsleuthburger asked on Rosewater's Tumblr blog, "Rather than focusing on a single ancient culture, what about using multiple ancient cultures in some grand battle? Imagine it! Ninjas vs Samurai vs Spartans vs Vikings vs Egyptians vs Persians etc." Rosewater replied:
Multiple influences in a single set? I like it. I’ll pass the idea along.
The 82 notes on the post so far have been full of excitement for the possibilities. User catholicchaoticist wrote "A Dantean prison plane with planar-diverse inmates would be a fun way to hit several notes at once," while alexcm4297 said "It could be done like Ravnica or Shards, where each culture has its own color combo. I really like this." In fact, the venerable prison plane and arena plane ideas both came up several times.
Indeed, as Ravnica itself is composed of the descendants of creatures kidnapped from other planes eons ago, this might be an interesting idea for the far future of Ravnica, where different extraplanar cultures each buddy up with one of the guilds.