I'm just saying this: Im playing, soaking up the atmosphere and sound effects, and suddenly its ruined by the big dumb footsteps of the "big daddy". Moreover, its old now. Ok, its been played well in the first two, but now its time to drop it for part III. Makes it more legendary.
Part III could be about visiting Rapture 5 years later, and someone else took over. With no little sisters, they get theri adam soley from sea slugs, and then have to have it processed at a certain "lab" run by a crazy splicer. You have ot collect many sea slugs (giving us much outdoor time), and take it to the lab as well. You have to negotiate a deal with the splicer running the lab on the adam. In fact, some splicers in the game will attack, and some will ignore you. Some of those you can make "deals with"...and some will keep the bargin, and some will turn on you. Interesting gameplay.
Mabey they could use my idea about Leprchaun splicers or something similar to that.
Get rid of the Bd/LS thing, and you have a whole new game

Introduce a whole new charachter. Stop using the same old stuff!