What I'd like to see:
-A final difficulty mode where the entire game is similar to Act V in DII in regards to random spawns of mob types. Everything would be extremely difficult, and the best gear in the game could drop from anything in that difficulty. Achievements could be earned by completing certain tasks in the difficulty (kill 100 fallen in 5 minutes in inferno difficulty, for example).
-I've seen a "survival mode" brought up very often on these forums ever since DIII was originally announced. Due to the popularity of the idea, they might have actually implemented it as "inferno" mode. That would explain the "hush hush," as it would be a pretty neat thing to announce closer to release.
-Extra dungeons that are completely randomly generated and have very loose ties to the story. There could be a mini-boss at the end of each one. Monsters could be more difficult than anything but the very last monsters in hell mode. Sort of like the little dungeons you had to do to get the organs for Ubers.
-It could be something really lame and anticlimactic like they call hell mode with 4 people in a game "inferno" mode. Seems doubtful with the lack of blue response to the inferno threads.