I'm making a single-player third person 3D adventure game, and I was wondering if I should fix the camera's FOV, or if I should create an in-game slider to change it. If I should create a slider, what should I limit the range to, and what should be the default?
For third person? Allow change of the camera distance, assuming you're also allowing the player to move the camera around. It's hard to sometimes get your bearings on where you are in 3rd, and you need that zoom and move camera function to help establish spatial awareness.
Go play other game and see what they do. In other words, make it something game players are familiar with already.
A very common design pattern is that the mouse wheel zooms the camera in and out, and right click drag moves the camera. Left click is for player interactions.