The core developers set the feature agenda for major releases based on Trac tickets, known problems, and user surveys. Planning for the next version doesn’t begin until a week or two after a major release. The development schedule and weekly meeting agendas are kept on http://make.wordpress.org/core. The weekly developer meetings take place in the #wordpress-dev IRC channel on the Freenode network. Anyone can join in the meeting, but they must respect the agenda.
If a new feature can be handled by a plugin, the developers are less likely to consider it for inclusion in the core. Core is for ...