What about Brothers Grimm? just saying, if Matt posted those names, means those are not the ones, he is just playing with the fans, and i tell u this, if they know S6 is the last, nothing makes more sense than bringing these 2.
I definetly could see Brothers Grimm being last big bads, probably working to resurrect or free the Black Fairy, who could be considered the original darkness, even before the Dark Ones, thus being the absolute worst of the worst.
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"If you go as far as you can see...you will then see enough to go even further." - Finn Balor