Necromancers of the Northwest has a Kickstarter campaign going to fund a sourcebook for the Elemental Plane of Fire for Pathfinder.
From the campaign:
Like many gamers, we love the planes, and want to see more content made to support them, and to help those GMs who want to run campaigns set in them. While there are numerous planes of existence in the various fantastic settings that have been associated with Dungeons and Dragons and Pathfinder, both published and brewed up at the kitchen tables of GMs worldwide, we feel that the elemental plane of fire is a great place to begin an examination of the planes. Not only is it an almost-universal plane, that will fit more easily into almost every setting than certain celestial heavens or hellish realms, the plane of fire also has quite a bit less written about it than many of these other planes, but is also positioned quite well for the sorts of conflicts that make a setting interesting and worth adventuring in.