Dirt Material layer. It uses UE4 precomputed AO as input mask and triplanar texture projection. As well it has procedural leaking effect based on object bounds. It is possible to override dirt color per material instance and add additional "contact" color, control dirt roughness, color. Add dirt effect to chatacters.
This material layer can be controlled globally trough special blueprint actor, but parameters can be as well set inside material instance and blueprint will modulate these values.
Next steps - Most probably moving from being material layer to be part of base surface material, reusing existing surface textures to reduce tex samples, using importance masks or height maps for more natural blending with surface textures.
Blueprint actor will control lighting and atmospherics ( image based distance fade/fog), global wetness level, water level's etc. Maybe even some simple background meshes.
Idea is that end user just uses right parent materials in his/her levels and able to control most of parameters from one spot and change level look just with few sliders.