64GB seems like a lot to ask. GDDR6 is expected to go upto 32Gb per chip, so we may see 32GB (8 chips / 256bit bus) or 48GB (12 chips / 384bit bus). With HBM, HBM3 will have densities that could support 64GB with 4 stacks, I don't think HBM3 will be ready in 2 years for consumer deployment nor would they put 4 stacks on the interposer for a console.
Yes.
XBX1 can do 4K native for some games with 6Tflops. 12 Tflops would probably be sufficient for native 4k with minimal improvements overall in rendering. I don't think that will sell. There needs to be a compelling visual reason to upgrade.
In 2020 HBM should be cheaper in mass production than GDDR and 64GB should be possible too. Before they use 32GB they will probably wait another year.
I see no problem with other native resolutions like 1620p or 1800p. Especially since interlacing introduces ugly artefacts in motion. More important is the used AA where I liked the TAA of AC: Origins very much.