From what we know AMD and Nvidia won’t be releasing 20nm GPUs ever, as the yields are so bad that it would not make any sense to manufacture them. It is not economically viable to replace 28nm production with 20nm.
This means the real next big thing technology will be coming with 16nm / 14nm FinFET from TSMC and GlobalFoundries / Samsung respectively, but we know that AMD is working on Caribbean Islands and Fiji as well, while Nvidia has been working on its new chip too.
This doesn’t mean that you cannot pull a small miracle in 28nm, as Nvidia did that back in September 2014 with Maxwell and proved that you can make a big difference with optimization on the same manufacturing process, in case when the new node is not an option.
Despite the lack of 20nm chips we still think that next gen Nvidia and AMD chips bring some innovations and make you want to upgrade in order to buy it to play the latest games on FreeSync or G-Sync monitors, or in 4K/UHD resolutions.