BioShock 2 is a FPS developed by 2K Marin and Irrational Games, released in february 2010 for PC, Xbox 360 and PlayStation 3. Users at 2K Forums noticed many beta differences in the early trailers of the game, especially in the “Hunting the Big Sister” demo:

• It would have been possible to explore areas from the 1st game (some of them underwater)
• Tenenbaum was going to be the player’s guide
• Splicers ran away in fear from the player
• Textures looked more like the original Bioshock
• You were able to carry more health packs and eve hypos
• The whole demo is set up differently. It starts as the normal game would, but the path and events are fairly different, including the final scene of the demo, that location, Fontaine Futuristics, is towards the end of the game.
• Looks like the drill recharges when it’s not used. In the final game you had to find drill fuel.

Also, as we can read on Wikipedia, the original story and gameplay elements related to the plot were changed / removed from the final game:

The story received major changes over the course of development, with two of the most important relating to the player’s character and the Big Sister. Initially there was only going to be one Big Sister who would continually hunt the player down throughout the course of the game and then retreat once she was defeated.

This Big Sister was written as a Little Sister who, as she grew up on the surface, could not leave the memory of Rapture behind and eventually returned. The reason for the change, as explained by Zak McClendon, Lead Designer for 2K Marin, “If you have a single character that the player knows they can’t kill because they’re so important to the story you’re completely removing the triumph of overcoming that encounter with them.”

Jordan Thomas explains however, “The soul of the original Big Sister character still exists, but in the form of somebody you get to know over the course of the game.” The other major change is that the player’s character, Subject Delta, is no longer the first Big Daddy, but rather the fourth prototype. He is, however, the first to be successfully ‘pair-bonded’ to a single Little Sister.

Also, according to Rockpapershotgun’s BS2 review the special edition’s art book shows lots of unused designs

I’ve already completed Bioshock 2. None of these things were in there – the Big Daddy with arms like tortured tentacles, another which looked like a spaceship on legs, the Splicer whose grotesquely mutated face had become a fleshy whirlpool, a hideous sea-beast halfway between a merman and giant phallus, a frail, frock-wearing Big Sister who carries her hulking oxygen tank around on a rickety hand cart… Where are they? What happened?

Thanks to Robert Seddon, Robert and Dr. Swank for the contributions! If you can find more differences in the early Bioshock 2 screens and videos, please let us know!

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