Personally, when writing a story I have a clear idea of the beginning and the end. Then I write the shortest possible way the story can go from the established point A to point B with a minimal cast of characters because this way at least I have finished the product even if I get tired of it later on. When I have the basic bare-bones game I expand it little by little, because you can always add things in the middle and be as ambitious as you like.

You need to make a strict decision and be confident about your story. Do you want linear story or branching story? choose one but remember that each choice has consequences.

Since you haven't finished any RPG projects, I highly suggest to choose linear story and make the game with it.

Try restricting yourself to only 2 or 3 decisions that will affect the story line.  It's important to understand the development cost of each decision - doubling the size of the project vs adding a short scene that'll influence a later scene (or even not do anything other than be a part of the experience).

my story suddenly appear while im working on my games completely without making any plans i know its not common to do so and maybe my games are not the story killers like rpgmaker games but for (old-school) fps games i came up with pretty interesting stuff lol but everyone will have a different opionion on my way of working and stories :)