A PostgreSQL major version is usually released once a year. For the next major version, which is 9.5, this probably will be in the second half of 2015.
As PostgreSQL is an open source product everything that goes on is available to the public. If you want to know what will be coming in the next release there are several places you might take a look at.

The PostgreSQL documentation

The official PostgreSQL documentation is hosted on the postgresql.org website. For getting the documentation for the latest release (9.4.1 currently) you just point your browser of choice to the following URL:
But in addition there is the documentation for the release currently in development. Just replace “current” with “devel” in the URL above and you’ll get the documentation for the next release:
Remember that the development documentation might be incomplete, wrong or even missing for some topics. But this still can be a source for getting an overview of new features that are in the pipe. The upcoming BRIN index for example is already documented very well.

The PostgreSQL Wiki

Another source of information is the official PostgreSQL WIki. Especially the development information pageprovides plenty of links, e.g. the todo list.

The PostgreSQL Commitfests

If you want to dig deeper in what is going on in development there are the PostgreSQL commitfests. This is where all the new features/patches/bugfixes are tracked. At the time of writing the Commitfest 2015-06 is the latest one and if you take a look at it you’ll see active patches in the following areas: bugfixes, documentation, clients, performance and so on. You can even see who is working on the patches, who will do the review of a specific patch and the current status of the patch:

The Hackers Mailing list

Another possibility (if you you want to get hundreds of mails per day) is to subscribe to the pgsql-hackers mailing list. This is where all the discussions around bugs and new features take place.
The mailing list can be browsed online, too.

Planet Postgresql

Last but not least planet postgresql is the feed and blog aggregation placeto check regularly. If anything interesting is happening in the postgresql world it will pop up here.

… and if you still hungry

… you can always checkout the latest git repository and dive into the source code.