Theros block is now firmly behind us, and R&D members, like the rest of us, are looking back on Magic's second top-down world in three blocks and doing their own postmortems. Mark Rosewater is one of them, and while we'll know his true feelings when the 2014 State of Design comes out in August, he's let slip what he thinks were the best and worst things about Theros - and hence what might be done again.
When robogles asked him on his Tumblr blog, "Out of all of the mechanics from the Theros block, which do you think was the biggest success in the big picture of Magic?" Mark gave an unqualified and definitive answer: "Devotion."
Out of all of the mechanics in Theros, devotion sort of breaks the most and the least new ground at the same time. While it is sort of just a reskinning of chroma, the Shadowmoor-block ability word, it's also got a lot more design space to be used in the future. It made the Gods mechanically possible, for one, including the multicolor ones, and it synergizes in a smart way with the hybrid mana cards from Return to Ravnica block. The multicolor Gods' dual-color devotion is just a small part of the (mostly) untouched design space: Besides varying the types of mana it keys to (snow or Phyrexian mana?) you can use devotion to set an infinite variety of variables.
Of the other Theros keywords, scry is probably the most likely to return - it's even been core set material in the past, and is mechanically generic and always useful enough to be brought back in any given block that needs a utility spell keyword. Heroic also seems to have some design space left - while bestow has an incredible amount of complexity and brings with it the baggage of enchantment creatures; monstrous tends to fit in "battlecruiser" style Limited sets; and tribute and strive are almost entirely out of the question.