A complex AI system for all the characters in a game, so that you can literally talk with them outside the usual repetitive scripts. Actual characters made experts of the game lore and so able to discuss about the world they are in. It's complex because right now it's very difficult to avoid errors and guarantee coherence, but it's an inevitable (and the most grand) step in the gaming future, which will allow in time to deal with world literally populated with lifelike characters, opening for a whole new dimension of gaming and phylosophical discussions. Interacting like that in VR, with the exponential freedom of spatial interaction that it allows, would be the pinnacle of possiblities, until full dive systems, if they will ever exist.
And for God's sake, no more floaty, weightless, sliding animations... Even the most photorealistic character look bad when it moves without weight and inerthia. REAL physics based animations will constitute a bigger VISUAL difference than any progress in rendering, in the next ten years. Same said for the physical interaction with the game world, which has to implement also anelastic reactions, bending ones and proper friction.
Oh, and also real open worlds (like the one theorized in Light no Fire) with actual physical laws applied to the game world, so that you can freely play with them. That's incredibly satisfying and in capable to open so many emerging forms of gameplay that any game that will manage to do so will enter hystory.
On science fiction possibilities, in addition to the full dive experience, also a quantum entangle-based internet connection to allow completely lag-free connections with MMOGs capable to hols lag free, with all the abovementioned complexities, thousands of players at once, that could have direct interaction at the level of an offline Virtua Fighter match, completely lag-free. That would open even full scale battles between thousands, but still with complete precise control for every player. Still far from being feasible.