It seems like just yesterday that graphics cards with HDMI 2.0 support started shipping. That specification was first released at the tail end of 2013, and supports 4K resolution at up to 60Hz. Nvidia baked HDMI 2.0 support into most of its second-generation Maxwell in 2014 and 2015, while cards based on the Polaris architecture were the first from AMD to support the standard last year. The HDMI Forum isn't sitting still, and it recently announced that it's working on a new HDMI 2.1 specification, which includes support for 4K at 120Hz, 8K at 60Hz, dynamic HDR, and variable refresh rate technology.
HDMI 2.1 is fully backwards-compatible with older versions of the spec. The maximum refresh rate for the 4K (3840x2160) resolution goes up from 60Hz to 120Hz. Refresh rates as high as 60Hz for 8K resolution are also included in the new specification. Dynamic HDR allows variation of "depth, detail, brightness, contrast, and wider color gamuts" on a scene-by-scene or even a frame-by-frame basis. The specification also includes an upgraded version of the HDMI's Audio Return Channel called eARC, which should be capable of handling object-based audio, audio signal control, and device detection.
The most interesting thing about HDMI 2.1 is a brief mention of something called Game Mode VRR, which aims to be a variable refresh rate technology allowing a graphics processor to display a frame as soon as it is rendered, rather than waiting for the screen's next refresh cycle. Whether Game Mode VRR is an adaptation of AMD's open FreeSync variable refresh rate technology or some new third VRR standard in addition to FreeSync and Nvidia's proprietary G-Sync, is a fact that remains unclear.
All this magic looks like it will require new cables. The specification mentions 48G cables capable of ferrying 48 gigabits of picture data from source-to-screen each second. The HDMI Forum says these new cables will be backwards compatible and can be used with existing devices, so gerbils won't need to throw away their HDMI 1.0-era upscaling DVD players when they get their 8K televisions in a year or two. The HDMI 2.1 specification is expected to be finalized in the second quarter of this year.