http://www.hollywoodreporter.com/news/girl-dragon-tattoo-sequel-whats-416621
Sounds like a lot of speculation, but still has some interesting info:
"David Fincher is in early talks to direct Fox's adaptation of the psychological thriller novel Gone Girl"
"The studio is said to be hellbent on reducing the cost of the next chapter"
"If Sony can't bring Craig back to reprise his role as journalist Mikael Blomkvist, the sources say the studio could write the character out of the sequel." (he wants a raise, they want him to take a pay cut)
"Sony would prefer not to be forced to commission a rewrite of Steven Zaillian's script" (expensive and busy writer)
"As for Fincher, a project insider says don't count him out of the running for Played With Fire."
"The only film seriously vying for Fincher's immediate attention is Disney's 20,000 Leagues Under the Sea, but the helmer and studio can't seem to find common ground on the budget." (Gone Girl is far from production ready)