PostgreSQL is a non-commercial, all volunteer, free software project, and as such there is no formal list of feature requirements required for development. We really do follow the mantra of letting developers scratch their own itches.
The next release of PostgreSQL is planned to be the 9.5 release. A tentative schedule for this version has a release in the fourth quarter of 2015.
While there are no formal requirements for each PostgreSQL release, there are several places you can look to find out more information on upcoming features: