I'm using an XBox One S, so if you have the xb1x this might not apply.

I went in to the Graphics options, and either turned off or all the way down all of the options except for anti-aliasing.

This reduced the load on the gpu enough that the game automatically increased the resolution. Before it looked to be 720p, afterwards I believe it was 1080p - but I have no way of knowing for sure. All I know is that it looked MUCH better.

I believe the biggest culprits are the lighting and shadow effects - there may be a middle ground where you can have this turned up a bit and still get the higher resolution, I didn't play with it that much.

Lastly, there's an option for something about "cool color" I found that keeping this enabled caused the colors to be very vibrant, but appeared over-saturated to me. Turning this off looked better, but that's subjective.