Subcon is the world of dreams. It was the setting for the American version of Super Mario Bros. 2.In Super Mario Bros. Z it is likely to be the location of a Chaos Emerald, as references to it have been made to it in the Anime-opening and Episode 7. More specifically, the world's tyrant, Wart, along with his Ninji followers were seen briefly in the opening and Kolorado stated that he wished to begin an expedition to Subcon. Furthermore, Kolorado's watch originated from this mysterious dream land, which is possible for him to go there again too.
It cannot be said for sure how someone accesses Subcon. In the Super Mario Bros. 2 Instruction booklet, Mario and company found a doorway in the sky. At the end of the game, however, Mario woke up, making it possible that the doorway in question was also in a dream.
Because of this, it is possible that Mecha Sonic, being a robot, can't enter Subcon. This doesn't necessarily make the Emerald (or other target) an easy win for the heroes as Wart and his followers are likely to be hostile and several other villains are capable of dreaming.
In the official Super Mario Bros. Z reboot opening, King K. Rool can be seen instead of Wart. After its release, Alvin-Earthworm confirmed that King K. Rool will indeed replace Wart in the new series and that Subcon had been scrapped. 
He said in a Deviantart comment that the heroes, after leaving the Omega Doomship, were planned to encounter Wart's Ninjis pursuing Kolorado and Goombella in the desert, and later the Ninjis would have escaped with one of the Chaos Emeralds. The heroes would have then chased them to a dimensional doorway to Subcon and encountered Wart.
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