Defending from sprawl isn't a particularly dangerous position if you can get hooks and control your opponent's body. There's not much you can do from that defense, however. Without a solid base, returning to a standing position is difficult, so generally the fighter on bottom will attempt to establish a base by closing the distance and returning to standup. The bottom fighter can also give up on the sprawl, hit the mat and turtle - but that will generally give the top fighter an easy opportunity to gain back control.
It's a tricky position to engage offense from; in a street fight a north-south position would be perfect for knees to the head, but those (to a downed opponent) are illegal in MMA. The top fighter does have more leverage for striking the side of the body, though, and the neck is open for cranks and chokes.
More commonly, however, both fighters will immediately attempt to transition from north-south. The fighter on top can move to their side to establish side control; the fighter on bottom will frequently attempt to turn over to sprawl and power to their feet.
Summary: Grappling positions are fluid and will change many times over the course of a grappling-intensive MMA match. Order them in a spectrum in your head, and look for effective transitions and transition defense. Keep in mind who's maintaining dominant position and for how long and you'll be prepared to know who won the ground war if the fight goes the distance.
Photo from OCMMA.com.