Here are several types of possible new content.

- Story extension after the main plot.
The traditional expansion. You keep playing your characters but you need to increase the max level and depending on how the story ends in the original game it might not make sense for the story

- Side story/quest during the main story.
Problem here is that you then earn more XP and are overleveled for the main story

- Additional Options
Not a story expansion but more Origin stories/companions/classes. Here no change with the level cap and XP are required but it might not be what some players want as they still need to play the same story yet again.