Oblivion is made in such a way that every "questline" (story) is selfcontained, with a very few exceptions. But yeah, apart from some talk in Towns about this Island, and maybe some new random sidequests (can't remember) nothing will in any way alterthe questlines from the base game.
It will sit inactive untill you chose to go there, and if I remember there is a level cap on it, so you can not even begin it before a certain level.
Now there was Knights of the Nine, which felt more forced onto you, but if I remember you could also ignore that untill you wanted to.
So every single selfcontained story (or questliens) will never interfere with eachother, or close off other questlines. At most you will be directed to where each of the major lines are so you can commence them at your wish.
And because I did not read your reply proper: Yeah this very much feels like it is set after the Mainline.
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