It appears that just yesterday, Square Enix filed a trademark for "Deus Ex: Human Defiance."
Considering how popular and successful Eidos Montreal's Deus Ex: Human Revolution was, it seems all too obvious that this is likely going to be the name of the sequel. It is also important to note, however, that Square is looking into having a movie made as well, so there's always the chance that this trademark is instead tied to the film.
I don't know about you, but I'm hoping it's for another game.
[Via]