Mark Rosewater, and to a lesser extent other members of R&D, have said many times that some planes are just not popular enough to bear revisiting in a block. While Zendikar and Innistrad are sure to get sequel blocks, for example, there are other locations associated with unpopular sets that would simply be bad business to reuse.
However, the usual rules never seem to apply to supplemental products, so when two of the new legends from Commander 2013 turned out to be from known, past planes (Jeleva, Nephalia's Scourge is from Innistrad and Prossh, Skyraider of Kher is of course from Dominaria), rampant speculation about fanservice-ish flavor references to other planes began. So what characters from planes like Kamigawa with small and passionate fanbases could we see? Let's try to guess:
Ulgrotha: The infamous plane of Homelands spawned some quite memorable characters - Serra and Baron Sengir among them. I'd say one of the Baron's many extended family members could make an appearance, but Jeleva already exists in this set as a new Vampire legend. (Funnily enough, the Gothic elements of Ulgrotha, its Vampire aristocracy, and the theme of a missing Angel goddess with an ailing high priest made some wits say Innistrad was actually "Return to Ulgrotha" all along.) Even better would be Zeki the undead reef buccaneer as a blue-black legend - the game needs more Zombie Pirates.
Kamigawa: Ahhh, the apparent favorite setting of all the commenters on Mark Rosewater's Tumblr blog! As a "legend world" the plane of Kamigawa doesn't lack for characters that never made it onto a card, despite the massive glut of legendary permanents in that block. The Ogre gang leader Boss Uramon always seemed interesting - though she may be deceased in the current timeline, continuity isn't important for a Commander set. Also, by the end of th storyline two characters were significantly changed - Hidetsugu had become the new All-Consuming Oni of Chaos and hence would be a great black-red legend, and Michiko had merged with Kyodai, O-Kagachi's child, and the Sisters of Flesh and Spirit, as they were hence known, protected all of Kamigawa - very green-white.
Mercadia: The topsy-turvy world where everything is for sale - this had so much potential, but was wasted on a single set in a woefully underpowered and directionless block. A couple of good legends could redeem it - The head of the sinister Cateran Mercenaries, the feckless magistrate, Ta-Spon the Ramosian executioner, or perhaps Ramos the renegade interplanar Dragon machine himself.
What planes and legends would you like to see represented in Commander 2013? Let me know in the comments.