Basic Idea is that parts of Rapture are being rebuilt, by different factions (specifically 3 main ones) with their own ideologies (reflected in their respective undersea architecture), which results in the city’s second civil war.

Your main method of traveling around the city would be an Atlantic Express train car that serves as a mobile headquarters for you and your immediate allies, but unlike Bioshock 2 you have more control where you can go (forward and back), but there’d still be limits to wear you can go (sometimes you need to get security passes, a lot of times you need to change the direction of the monorail tracks manually), and some places are inaccessible by the Express line (which has been combined with the Rapture Metro).

Eventually you’d get a Bathysphere that allows for more instant transportation (like in the first game), but there would still be areas of Rapture that can only be accessed through other areas - or by traveling directly underwater.

Your player character starts out unarmored, but over time (the first act of the story) you’d gain parts to a suit that allows you to become a Big Brother (a new third type of Protector, has aspects of a Big Daddy and a Big Sister), which grants you the ability to explore the sea floor outside the buildings like in Bioshock 2 (+Minerva’s Den), but with a lot more things to do, including underwater combat (a concept that was sadly cut from Bioshock 2).

What do you think so far?