This just boils down to a limitation of your imagination, and should not determine where a franchise should head for the developers in charge.
Like many people, when they see the incredibly realized and unique location of Rapture, you assume "nothing can top this! Hell, it'd be hard for anywhere to even equal it!" And it is true, Rapture is one of the truly brilliant gaming locations ever devised.
But to say there's nowhere else that could be as creative or inspired? I think that's a little bit much. Now again, I am not the idea maker, but I can think of a few other places. Interesting locations for moguls and their twisted morals and philosophy.
Again, I'm not making money, but the world is filled with amazing architectures and styles to choose from. We're an incredibly diverse universe and there are so many possibilities it's insane to think "well, Bioshock has hit the peak. No other ideas are worth trying."
Take a look at some diverse architecture:
http://www.newhouseofart.com/wp-con...ern-float-ecopolis-lilypad-architecture-5.jpg
http://www.thecoolist.com/wp-conten...r_by-ashton-raggatt-mcdougal-architects_1.jpg
http://trishamillerthailand.info/blog/wp-content/uploads/2009/10/UN_Memorial_modern-zen.com.jpg
I'm not making a statement on whether any of these are good, just a statement on the diversity of architecture. There's a plethora of legitimate styles to choose from, and amazing locations. And even better, this is gaming... where you can invent your own.
It could be a floating city, ringed by mist among the mountains, overrun with flora and fauna. It could be a decaying temple, a portrait of hedonism and religion gone mad. Who knows? It could be anywhere. Just, it can't be Rapture again.