At this point, we only "officially" know of a few cards in Theros, and one of the keywords is still somewhat of a mystery (although reliable sources point to the last, returning, keyword being scry). We've also gotten some flavor hints at many of the characters and some teasers from Mark Rosewater. Now, another seemingly reliable source on the MTGSalvation.com Rumor Mill forum has emerged to tie several of those things together into cohesive information. According to new user Trollcreature, the Sphinx legend Medomai is the one with the "Power Nine saboteur ability."
Specifically, the creature that Mark Rosewater referred to in his State of Design 2013 article is "a U/W Legend named Medomai, and his deals combat damage is a Time Walk at a price." It has since become clear that this could mean either the effect is not free, and has an actual cost; or is simply conditional. Repeatable extra turns, in order to not be infinite engines with themselves, have to cost something other than mana, that doesn't refresh every turn and requires effort to renew, as with Wanderwine Prophets and Walk the Aeons. Time Walk seemed like the least likely candidate for this effect, and Trollcreature's words were taken to be at best idle speculation and baseless guesses - but they were given credibility because they referenced Medomai before the character was known from the recent Planeswalker's Guide.
If the returning last mechanic is indeed scry, Medomai could wind up looking something like this:
Medomai, True Visionary 2WU
Legendary Creature - Sphinx (Mythic)
Whenever Medomai, True Visionary deals combat damage to a player, scry 2, then reveal the top two cards of your library. If they are both enchantments, take an extra turn after this one.
In addition, Trollcreature wrote "Also a 1 mana white enchantment Swords to Plowshares, and Thoughtseize is in this block." "One-mana white enchantment Swords to Plowshares" probably refers to a Journey to Nowhere that costs only W and has the Swords drawback, rather than something that exiles enchantments and gives their controller lifegain, since enchantment removal can already cost only that much without a drawback. Thoughtseize, meanwhile, is the "card that could have been in Modern Masters," which R&D excluded as to not make Modern Masters an all-goodstuff set, so to speak. Thoughtseize is currently a $60 card, and the threat of a reprint will probably drive prices down.
Finally, the dual land cycle appears to stem from Nimbus Maze, which is consistent with the notion that there is a futureshifted card in Theros, and R&D's desire to make Standard mana bases less diverse after Return to Ravnica's "Christmas land" situation.