Recs.
Updated
FPS Creator is a game engine specifically designed to create open-world FPS games.
Specs
Pros
Pro Feature priorities are chosen by the community
The Game Creators team frequent the forums and ask the community for feeback and suggestions both there and via an open poll on the website. They have a great history of reading suggestions, viewing screenshots, and generally working with the community.
Pro Active and helpful community
Users share ideas, help, and advice through the official forums. The community also provides a wealth of free assets, and the FPS Creator Store sees constant additions.
Cons
Con Currently in beta
FPSCR is currently a beta product with features potentially important to a specific project still in the roadmap. Obviously this is expected of a beta product, and new features are constantly being added, but should be taken into consideration when choosing a game engine for a game right now. To see what's currently implemented, being implemented and in the roadmap go here.
Con Can only do outdoor games currently
No customizable buildings, doors, elevators, switches.. can't make holes in terrain, caves, underground structures. The outdoor is limited too: can't do rivers, caves, no atmospheric effects like wind, snow, rain, snowstorm, windstorm, sandstorm, rainstorm. only one basic water plane.