I really, really would not want a 3rd-person camera at all. I've noticed that if a game has a 3rd-person camera, then the devs often make the indoor environments much larger than they would be in reality. They make the ceilings much higher, and the doorways much larger to make room for the camera. It makes the indoor environments look ugly and weird. One particularly bad example of this phenomenon would be GTA4. It's been a long time since I played Thief 3 so I don't remember to what extent this phenomenon affected Thief 3.
I would want Thief 4 to be like Thief 1 and 2, same as most people in this thread it seems. A way of physically interacting with the environment like in Amnesia would be really cool though. Dark Engine-style movement with Amnesia-style object physics would be great. I would want tougher/smarter guards and a better ladder climbing/mantling system too but that's about it.
I think it's important that they keep the sense of wonder in the game.
The Pagans and Keepers and Hammers can return, but the game still needs that sense of the unknown. I think it's what made Thief so great; you didn't know what to expect. Those moments like the first time that zombie stood right back up and attacked you, or you had to traverse past these menacing, dormant trees that croak loudly as you approach... those moments are memorable.
I understand that we all want the familiar and might feel like anything new that's added would never reach the quality of LGS, but I honestly think it'd be a real shame if nothing new was added to Thief 4. The cradle is highly-regarded for this very reason. Thief 4 needs to do what the original games did, play on your fear of the unknown to build suspense. In order to do that, Eidos Montreal needs the liberty to create the unknown in the first place.
Yes! Sense of wonder!
For me, the familiarity I want is to be able to move around wherever, in the way I'm used to. I'm a pretty hopeless gamer really but I always felt so comfortable in Thief TDP and MA. I didn't have to think about how I was doing something, I just did it. TDS took me out of myself and I felt I was always faffing around with trying to work out how to do something or what those stupid icons meant and how did I use them. I have a sense of failure because I only played a couple of missions. So, that's all I want, just to be comfortable and not have to think about how to do something - just be able to do it.