Koki's name is listed in the cast for the Ooku movie that comes right after the conclusion of the drama series. At the moment there is no official confirmation on the role he will play, but there is speculation he will continue to play Gyokuei (his character from the drama) but just an older version. This is in contrast to Sakai Masato, who stars in both the drama and the movie, but plays completely different characters. The movie is based off volumes 4-6 of the manga, and we know that Gyokuei is in those volumes as well.
Ooku Vol.5 Summary (Amazon): [Possible spoilers?]"Gyokuei, who first arrived at the Inner Chambers as a young novitiate monk, has risen to become one of the Shogun Tsunayoshi’s advisors. He hopes to protect the shogunate the same way his mentor Arikoto once did. But the numerous intrigue of the Inner Chambers-and the shogun herself-are pushing Gyokuei’s patience and cunning to the breaking point."
Tokyograph - "After the TV series ends, the “Ooku: Emonnosuke Tsunayoshi Hen” movie adaptation will be released in theaters on December 22. It will cover volumes 4 through 6 of the manga. Sakai will play Emonnosuke, an “ooku” member who resembles Arikoto, and Kanno Miho (34) will play the shogun Tokugawa Tsunayoshi."
ETA 3/11: Ooku ~Eien~ will be released on DVD/Blu-ray on 2013.06.08. The original soundtrack for the movie went on sale 2012.12.19.