The original multiplayer takes place over the course of a year, from the beginning of the Civil War to the beginning of BioShock, and records the decay of several Rapturians over the course of that year. My version of the multiplayer would take place over the course of ten years, so it would range from the Civil War to the events of BioShock 2, which would mean that the final cutscene would involve something to do with Subject Delta.
Now, the only problem here is how to make Big Daddies fighting each other all the time look convincing. My idea for the plot would be the following:
1) Player takes control of their Alpha Series Big Daddy in some part of Rapture. This is a singleplayer experience, so no co-op or fighting others yet. The objective for the player is to watch over their Little Sister. They will be able to use the Drill, the Rivet Gun and Electro Bolt.
2) After some time, the player gets ambushed. A huge fight ensues.
3) No matter what happens, after a while the player blacks out from the damage they take. When they wake up, they find themselves restrained by two Splicers while a third (in another room) rips the slug out of their Little Sister. A group of about five Splicers stands guard in the middle of the room.
4) The moment the Little Sister dies, the broken bond makes the player's Alpha Series go berzerk. They manage to break free and proceed to kill all the Splicers (this is done by the player).
5) After killing everyone in the room, the player proceeds to break down the door. The moment they do that, they get hit with Electro Bolt by the Splicer inside the room (whom the player cannot identify clearly) and black out again.
6) The player wakes up in a filthy cell in Persephone. There is a recording machine in a corner which holds the player's messages. There will be one message from the Splicer who killed your Little Sister, telling you that the only reason he didn't kill you was because "his boss" thought you could be useful. He also tells you to go out and fight, which opens up the multiplayer.
Basically, in the eyes of the player, every other player is different to them (i.e. they didn't go through the same ordeal), and those who aren't in their team are ordinary Alpha Series who don't work for "the boss". This means that when the player joins a team, they will always see their own team as "Us" and the other team as "Them".
At the beginning of your multiplayer career, the Splicer who killed your Little Sister (who will act as your mentor and drop a message on the recording machine every time you rank up) will be initially hostile, and will look down on you. As you level up, however, he will progressively warm up towards you, and by the end of the multiplayer career he will treat you as a good friend and respect you.
The following paragraph describes the endgame, so if you want to keep the feeling of suspense floating in the air, don't read. Nonexistent spoilers lie ahead:
Once the player reaches rank 100, "the boss" will reveal themself and will issue a direct order from the recording machine for the player to go and kill Subject Delta. Your mentor will then come in person to take you to the Atlantic Express train which you use to enter a match. At this point, you will be able to either kill him or let him live.
Spoilers end here.
So, whaddya think? Is it good, bad, a little cheesy perhaps? Tell me how you find it.