The release date still has not been confirmed but for season 2 of Haruhi (In short) the plot is going to be based on the fourth novel/manga for the series. its called "The Vanishment of Haruhi Suzumiya"
Heres a sneak peak of the plot thats most likely going to be adapted into season 2. The plot is from the 4th novel:
Spoiler
In this novel, the story takes place from
December 17until
December 24 a month after the cultural festival. On
December 17, everything is normal—the SOS Brigade plans to have a
nabe party for Christmas. But on
December 18, Kyon arrives at school to find everything has changed—
Haruhi Suzumiya is missing, and Ryoko Asakura is a normal student. Kyon is the only one who notices anything different. Nagato is an ordinary human, and Mikuru does not recognize him. The only clue is a bookmark left by the alien version of Nagato, which leads Kyon on a quest back in time, where he interacts with the storyline from "
Bamboo Leaf Rhapsody", trying to sort out the mystery and return to his own time line.
Sounds pretty strange and random to me but none the less, I look forward to it.