The Missing link fills in the empty space
At the beginning of The Missing Link, Adam is stripped of his use of his mechanical augmentations (and all other augs) allowing the player to remake Adam with up to 7 beginning Praxis kits, minimum of 5 Praxis kits what is nice, because this will carry over to the rest of the game.
The overall story is more improved, more backstory and more details of the world.
Th boss fights are alot harder then the rest of the game, this is because the game developers did this and not some koreans.
so by the add on this will give you more gameplay about 5 - 7 hours