I think that characterisations always depend on the story. A certain kind of a story requires a certain kind of Sess or Kagome or Inuyasha. No matter whether AU or Canon, you have to think of Sesshoumaru's history, the things that have made him who he is. Why does he appear detatched and cold? Is that who he truly is inside out, or is that only a facade for the world to see? What are his strengths, what are his weaknesses? How would he react in a given situation?
You said you already have ideas, so think about some of these questions to see if the ideas would work out.
There are certain things you know about Sesshoumaru from the canon series Inuyasha. In addition to that there are certain trends in the fandom, common and shared opinions about his personality.
But these only guide the way, creating the rest, adding in the depth and molding a Sesshoumaru suitable for the story you have in your mind is all up to you. I believe in the end Sesshoumaru's personality is what you make it to be.
Sorry if I wasn't much of a help, but I hope at least these pointers gave you something to think about! ^^