Sonic Team head Takashi Iizuka has talked briefly about how they decide to plan out the releases of Sonic the Hedgehog games. They put their tentative plans on a roadmap of what is planned for release each year and they then work out how much money is needed to be allocated to each project to make them reality. At present the roadmap is focussed on Sonic Racing CrossWorlds.
“First we create a roadmap of Sonic titles and think about which game to release in which year. We then allocate the development resources needed to develop those titles. So this year we’ve strengthened the racing genre, and we’re currently planning what to do next on the roadmap.”
