I´m coming from non realtime rendering and I wonder if some of the features a renderer offers could be implemented in UE4. Please excuse me if they may allready exist and I´m just not aware of them.
GI Override Material
A second slot in the mesh properties wich overrides the material when building the lighting only. Attached is an Image of a practical example. I created a tinted glass shader that can neither cast any shadows nor specifically colored shadows. So in this case I used a standard translucent shader to get the shadows and change the material afterwards to my new shader wich gives the result you see. This could be used in various occassions. Like having a less saturated material to prevent strong bleeding. Or in reverese increase the color bleeding with a more saturated material. Or to speed up lighting calculations and decrease the GI splotchiness by using a single colored material for lighting calculations instead of texture.
Invisible Reflection Objects.
I´m not sure if this is possible at all, cause of the way screenspace reflections work. But it would be great especially for Product Visualisation to have objects that reflect in other objects but are not visible to the camera. In product viz you want to have highlights and reflections to happen in a very specific place. For example the side of a car. In non realtime rendering you sometimes place invisible reflection planes very close to an object to precisely control where a reflection is happening. I know HDRI reflection are possible but you sometimes have just to less control over where an effect happens.