The ever-resourceful folk over on NeoGAF
have discovered a brand new trademark filing by publisher Square Enix today, claiming the name Deus Ex: Human Defiance via OHIM, the European Union's Trademarks and designs registration office.
We've verified this one, and it's legit. Having being registered for Square Enix by the same firm as 2010's Deus Ex: Human Revolution. Here's the search output from OHIM:
Speculation by the GAF crowd seems to be split between the title representing the rumoured Wii U version of Human Revolution, or the expected sequel from Eidos Montreal, and we're inclined to lean toward the latter.
Eidos Montreal are known to be working on Thief 4, however it's a large studio with multiple teams. So a couple of years on from the successful original, and with announcements for the next console generation gearing up, the timing seems right.