Nobuhiro Nakayam, the producer of the Shana project, was quoted from Gakken’s Animedia that:
he is aiming for the finale of the storyline, but before the staffers can do the third series, they must delve into the background of the characters and their deep relationships. According to Nakayama, that is why they are producing the Shakugan no Shana S original video anime project first.
The first Shakugan no Shana S OVA will go on sale in Japan on October 23.
Source: Anime Nation