I would like a game that takes place as a prequel to the other two, with you witnessing Rapture's descent into madness first hand beginning on New Year's Eve 1959. Seeing Rapture for the first time as a dilapidated city was amazing in the first game, but a third game just wouldnt have the same feeling to accompany it. Instead, imagine your awe as your chracter emerges blinking from the submarine into a thriving, alive rapture for the first time, with crowds of people, lights shining out into the ocean, artistic fountains pouring glistening water, and wonderful displays of unheard of before genetic alterations. Then, imagine your terror as the lights cut out a few levels later and you hear the first gunshots, screams, breaking glass, and rioting moving toward you in the darkness.
As far as I'm concerned, this is the only place Bioshock has left to go.