Masashi Kishimoto revealed in an interview with Fuji Television (translated by a fan Yagami1211) that Shikamaru’s conception occurred while he was writing the Chunin Exam Arc. When he asked to flesh out the exam process, he had the idea to create a character that would outmatch his classmates. He wanted Shikamaru to be the only person in Naruto’s generation to pass, and though he is the only one officially named Chunin, the test is interrupted by the arrival of Orochimaru. Shikamaru has obviously proven to the higher-ups that he has what it takes, but it is disappointing not to be able to see him at his best.
That isn’t the end of Shikamaru, though, and Kishimoto had many opportunities to flesh the ninja out. He shines during his battle with Temari (whom he eventually ends up marrying), and he must deal with the trauma of losing his mentor, Asuma. His intelligence gives him an edge over his enemies, which helps him on his quest to get stronger and be worthy of the Nara name.