What Apple needs to do to go UHD

1) License Dolby Vision for HDR. (This is optional, but I imagine Apple will push DV when all is said and done if they care about quality.)
2) Re-enable HEVC decode in future shipping silicon (likely in an existing SoC).*
3) Support HDCP 2.2 output.
4) Launch UHD iTunes. Apple would certainly have been working on encoding a UHD library and providing the necessary infrastructure around that for a long time.

*It's unclear if Apple has gotten any further with its licensing negotiations around the HEVC Advance issue. Perhaps there is finally licensing progress? Otherwise, AVC it is. I'm not sure, but I don't believe the UHD Alliance mandates HEVC encoding. AVC encoding would certainly be quite burdensome on bandwidth, and Apple has waited.

I will amend this post if I find any further information.