The century is still young, it will be VR/AR by the end of the century. VR is only at the point where early 3D was, when 2D games were still much better.
For the first decade it's definitely online gaming. It was there in the 90s, yet matured in the naughties.
Last decade I would say digital/indy games. Also already there in the naughties, yet last decade digital indie games broke through everywhere.
This decade, dunno. VR tech isn't ready yet for the main stream, nor is ray tracing to universally take over from baked lighting. Maybe this decade will be remembered for device agnostic gaming, cross platform, continue anywhere. But that's also not quite there yet. 6 years left anyway.