Guilty Crown: Lost Christmas is a visual novel spinoff being put together by Nitroplus (whose prior co-productions include the VN of
Steins;Gate and the LN series of
Fate/Zero). It is set for release in Japan in 2012, for 4800 yen.
At present,
few details have surfaced for the game; it is reported to be set in 2029 (the time of the "Lost Christmas" outbreak itself) and include an alternate protagonist, Scrooge (who, like Shuu, seems to also possess a Void Genome) as well as Carol (the lead heroine, who looks supiciously like Inori) and Present (a mysterious girl who fate seems to be intertwined with the leading pair).
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Sorry, I couldn't find too much more to go with right now.