I think for UE4 Nvidia VXGI looks very promising: HugeDomains.com
I foresee that any precomputed GI, light probe, reflection probe techniques are useful… but only for the next five or so years and certainly in the meantime for XboxOne and PS4.
However for PC-level hardware eg. Nvidia Maxwell, Pascal and beyond, especially with VR, from 2015-2025 game engines will have to move to full realtime GI… Which I think they will, since lighting is the key factor to the feeling of realism.
You can pretty much get realtime raytracing with Octane and a huge number of GPUs: Octane Render: realtime ray tracing - fxguide
Within 10 years the appropriate tweaks will bring that quality to realtime gaming on consumer-level hardware.
To sum up, I predict UE4 will be the last “non-fully-realtime-GI” Unreal Engine 
UE4 Lightmass already produces (for a “gamer” like me) jaw-dropping GI (albeit not fully realtime GI etc): Unreal Engine 4 + ArchVIZ by Koola - 3D Architectural Visualization & Rendering Blog
UE4/5 with pure-fully-realtime-GI …Well, we can say bye-bye to the physical world. Just in the next 10 years. Very scary.
PS Don’t forget the advancements in CPU and GPU physics. As many have noted, it’s not so much the polygon count and so on but physics and lighting that “suspends reality”. Microsoft’s “cloud physics” indicates how this will be easily done on consumer-level CPUs and GPUs by 2025: Microsoft Cloud Gaming Prototype (Build 2014 Xbox One/PC) - YouTube
PPS Eventually “voxel point cloud data” game engines will be out too and that will add to the hyper-reality of life in 2025: - YouTube
Enjoy reality while you can, reality is going bye-bye