Besides the 5 published PC adventure games by Benoit Sokal (Amerzone, Syberia, Syberia II, Paradise, Sinking Island), there are two known possible games, which were planned, but not realized.
One of them is "Aquarica":
Aquarica was born from a collaboration between Benoît Sokal and François Schuiten. Initially, it was a global project that included both a film and a video game. The video game would have been an adventure divided into episodes, downloadable from the internet. The film would have been a feature-length in synthetic images, produced by White Birds in collaboration with Duran Duboi. The projected budget was about 20 million euros according to White Birds marketing manager Michel Bams. But when White Birds closed down in 2010, there was little hope for this project to see the light of day.
After almost 10 years it looks the movie is finally moving forward.
Look here for detailed informations about the plot and a big image gallery:
https://www.altaplana.be/dictionary/aquaricaThe second not realized game is "Broadway":
It is rumored that Sinking Island will not be the last case for private investigator Jack Norm. What can you reveal about any future follow-up project?
The sequel is partly written already. It is called "Broadway" (it’s a codename) and the action takes place in a Broadway theater where a crime is committed during the rehearsals. All we have to do is find a publisher...
http://www.adventureclassicgaming.com/index.php/site/interviews/361/D'autre projets en cours ?
Michel Bams : ... Notre autre projet, c'est le prochain Sokal qui est en production chez nous depuis maintenant deux mois et qui s'appelle Broadway. Niveau principe, c'est dans la même logique que l'Île Noyée. C'est donc une investigation policière dans laquelle on utilise un assistant personnel. Mais à la différence de l'Ile Noyée nous le développons en même temps à destination du PC, de la Wii et de la DS pour une sortie prévue fin 2009.
Translation:
Other projects?
Michel Bams: ... Our other project is the next Sokal, which we produce for two months now and it is called Broadway. Level principle, it is the same logic like Sinking Island. This is a police investigation in which a PDA is used. But unlike Sinking Island we develop at the same time the PC version, Wii and DS for a release in late 2009.
http://www.jeuxvideo.com/news/2008/00029621-nikopol-interview-michel-bams.htmSo it seems that "Broadway" was in full development for at least 2 months in 2008, but then was cancelled by the publisher for unknown reasons.
I still hope that one or both games will be produced in the future, maybe if Syberia III is a big commercial success.