For me it has to be physics and polygon count.
Graphics have come a long way over the last 10 years but physics haven't had any where near the same amount of effort instilled on them to improve.
Half Life 2 really pushed physics to a new level in terms of integration and I'm struggling to find a recent game that set's a new standard for physical interaction with the player and world.
It's no use having graphics that try to look like the real world if they don't behave like the real world.
Horizon: Zero Dawn is beautiful but an Horizon: Zero Dawn with a state of the art physics engine with every object in game having physics would be mind blowing!
And.........Polygon counts.........nothing needs to be said here...........when DX11 released with tessellation I genuinely thought it would be the end of flat ground/wall textures and hard edges in graphics......and yet we still have them. Crysis 2 had fully tessellated walls and they looked awesome but aside from the odd game doing it there's not really been much effort gone in to it.
I would love to finally see the end of flat ground and wall textures.
What would you like to see an improvement on?