A poster containing an unannounced Nendoroid Hatsune Miku in Family Mart (a.k.a Famima) themed outfit was spotted by a tipster earlier today. It seems that Nendoroid Hatsune Miku: Family Mart version will be given away as one of seven prizes in an upcoming campaign by the store.
No other info has been announced yet, especially about the optional expressions, parts, and accessories. However, if you who can read Japanese, you might want to visit the source page for more info regarding the campaign. ^^
(Thanks for the news tip, Ryan Cheang!)