Doom: The Golden Souls

Doom : The Golden Souls is a groundbreaking hub-based mod, heavily influenced by one of the greatest games of all time: Super Mario 64.
To progress through the game, you must collect "Golden Souls" in order to unlock doors in the main hub, a big mansion. You can access any level through the paintings on the walls.
In order to explore every single part of a level, you may even need to backtrack in an older painting, after finding an important item.