I'd prefer simple story expansion, no new class, no new races, no need to rework the base game to fit the new class/races - although some aspect of base game do need a bit of touch up.
It can start in the middle of the game during House of Hope raid, the hook is already there.
Completely agree with this. No clue how much work it is for them to add new classes, races, sub-classes, etc. But I'd rather get new story content than new classes or races to play. I also imagine a new origin character would be way too much work for the return they would get - but maybe they could introduce a Jaheira/Minsc type character later in the game during whatever the new DLC is as well.
I also do not mind if the story itself is 'self-contained'. Meaning that it has little impact on the main story or events of the game. You just get teleported somewhere else during the main story and there is 5-20 hours of content or something like that in some other new area. I think it would even be great if they churned out multiple 'smaller' areas... similar to the size of the Underdark area or Grymforge with their own little activities and stuff like that.
The one thing I know I do not want is 'after the ending' content. I hope they leave the ending as is.