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 it cares 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.

Update

As became clear at WWDC 2017, Apple did resolve its HEVC licensing issues.