UE4 has started to bridge the gap between realtime game engine rendering and offline rendering, one of the things that I find has some room for improvement is lights.
Light Linking- have the ability to make a light include/exclude an array of actors, excluded actors would not recieve any lighting information from that particular light.
Render diffuse/specular/shadows independently- Having the ability to render JUST the diffuse, or JUST specular, or JUST shadows is incredibly powerful for great lighting. currently all lights have to have a diffuse power, so rendering shadows only is not possible also rendering just a specular highlight is not possible.
Shadow color- being able to modify each lights shadow color would be great, its a cheap trick to fake GI or add some cool visual effects.
Volume lights - Ive tried to get this to work, but my programming seems to be failing me. basically have a light shine to its fullest, and once it hits the bounds of a component(cube/sphere/capsule) it would not illuminate beyond that. This one isnt useful all the time, but it has some benefits on occasion.
Im not sure of the feasibility of these with a deferred renderer, but I think they could help a lot. Alternatively if you CAN do any of these things currently, please let me know because I havent been able to figure it.