If the No Man’s Sky devs made an engine it would look alot like Crysis. Great for what its built for AND if you know how to use it. It would take a decade or more for them to reach even a fraction of the flexibility and general game designing power that UE4 has.
Theoretically you could make No Man’s Sky in UE4. You have the source. Good luck with the math though. And probably rewriting the renderer and who knows how many other tools and systems. Tons and tons of work.
I am more interested in the idea of what No Man’s Sky has achieved. Applying procedural concepts to everything. I want procedural music, terrain, meshes, characters, game systems, etc, etc. You can build in tons of complexity in UE4, but it takes a lot of work and time. Pays off in the end though.
For UE4, that means more procedural access to systems in general, preferably from blueprint. Complete blueprint/procedural access to landscape, world browser, and every other system there is.
In the end, there is always C++ of course. With enough work I am pretty sure you can make anything in it.