I am curious who everyone thinks is coming. It could be just about anyone since Jacob can bring people to the island. Whoever it is though, it definitely scared The Man in Black. I have a couple ideas.
1. Charles Widmore and company. He was always talking about returning to the island. It appears that Ben is on The Man in Blacks side so perhaps Widmore is on Jacobs side. Widmore has been looking to return and maybe Jacob finally allowed him to.
2. The survivors who are at the Temple. Since after Jacob said that, the survivors warped to the present. Plus Jacob had a direct involvement when he talked to Hurley and told him to help Saiid. I do not know though why the Man in Black would be so afraid of them since he has already met all of them and terrorized them for a long time on the island as the Smoke Monster.
3. The alternate timeline people. They are a few years in the past, however perhaps they will return to the island. Once again I do not know why this would scare the Man in Black.