Epic did create a slot in the PPV to allow for external GI and reflections plugins, and there are many corner cases where lumen isn’t as robust as other potential solutions.
I’ve followed the engine pretty extensively on Github and have been pulling down and testing _main builds. Real-time GI, especially a general-purpose solution like lumen, is in its’ extreme infancy. I’ve seen modules get taken apart and rebuilt, certain feature paths added, removed, rolled into a different feature set. In EA, they had support for long-range GI via a reflective shadow map, and that got completely pulled out and replaced with the far-field (with no long-range SWRT solution left, to my knowledge).
So essentially, props to you, I think a more tunable lighting pipeline is always a good thing, but it could be a heck of an undertaking to not have the engine get too far away from you.
Although much of the GI and reflections pipeline is solved, I remember Daniel Wright talking about wanting explicit sampling for emissive surfaces, and the strata team looking into colored shadows.