I just ran into at least TWO websites that strongly hint that Epic Levels will be coming to 5th Edition D&D in 2017.
Firstly, there is the Miniatures website that tracks and catalogs ALL of the WizKids minis releases. One of the new minis releases in 2017 is the Epic Level Starter Set (Icons of the Realms).
So if there are indeed "epic level themed D&D minis" in the pipeline, we can guess that an epic level sourcebook is going to come out in the near future.
If that's the case, I'm guessing there will be an epic level storyline to coincide with it, probably something along the lines of going to the various planes....hmm maybe Planescape.