I think they should optimize the existing engine. As some users previously posted, even $2000+ PC's are unable to run a solid 60fps (god forbid 120+ fps) at max settings. The DX12 mod does help (especially with 1% minimums and quick rotations of the camera) but actually having dx12 built in, and optimizing the existing systems for current hardware would make a huge difference.
Then they could work on increasing texture resolution, draw distance, lighting, shaders, and animations to a higher level. I personally really enjoy the art style and don't think it needs drastic changes.