Special Jounin are ninjas who have gained high Jounin-level skills in one area of ninja arts. These are ninja like Morino Ibiki, who is highly skilled at interrogation and torture. Ninja attain the rank of Special Jounin through official appointment.
There is a high chance that Shikamaru could be promoted to Special Jounin for completing this mission. Shikamaru shows great intellectual and some killer battle tactics and he never puts a friend in danger. He has great leader skills and he never take risks that are too high.
Although Shikamaru does not have much stamina and chakra, and his fighting skills are average at best, he excells very well in other areas. Although he might not become a full Jounin yet, but I'm actually almost positive that Shikamaru could become Special Jounin right after the mission. He has time to become a Jounin later in life when some of his weaker skills become a little stronger.