It does sometimes seem that we see a lot of posts about creating a game, with details about the story, the world it is set in, (and you have even included the prices of the DLC which is new 
). There’s very little about the actual gameplay.
Play around with the Unreal Engine first. Just create small levels and see which parts of programming you enjoy doing. Remember, what started out as a game about Nazis and slugs on an island, became BioShock. The things you have listed will be the first things to change. (E.g. Different names for the main character? Just give them the same surname, have everyone call them that and you have saved a load of fuss and bother with the voice-acting).
Good luck!