The expansion this year (Lightfall) is the lead-up to next year's expansion (The Final Shape) which will be the end of the current 10yr Destiny story. The game will continue beyond that but nobody knows how.
Thanks now that you mention it I have seen/heard The Final Shape mentioned before.
Not that I will probably play it but I always find it interesting what they will decide and how to continue after that. Business decisions always intrigue me