Yeah there's definitely a new sound guy, I recall reading that. Though the first known media event about the game was him giving a talk about some new procedural ambient music stuff that he was working on for the game, it doesn't sound like he's treating it lightly (hopefully he pulls off the same amazing ambience).
While I loved loved loved the gameplay (particularly of the first two), I'd probably still be content if they took it in a different direction, so long as it was still Garrett in the The City. Otherwise, if they retained the same gameplay, but with a different character, I'd probably still be content - so long as they retain one of the two things which made it awesome.
God I miss The City, it was so... terrible.
edit: Oh, and they totally need to bring back the amazing cutscenes, Thief 3's weakest point imo was the inconsistency and cartooniness of those (though the trial and gammal reveal weren't bad). There were some amazing tiny tunes which partly made those cutscenes somehow so awesome (hope you guys played
thief gold with the extra interspersed stories), so I can definitely see it being a loss without the same music guy.