Moving right along, I added Ion's quote, and a few other developer quotes to the OP. I feel that Blizzard stating on multiple occasions that they're open to a third spec, and that there's even room for an Evoker 3rd spec is quite relevant to this thread.
EDIT: Going over some of those developer comments, especially Morgan Day's a few weeks ago, I do wonder if Blizzard is truly opposed to a tank spec for Evokers. If that's the case, we're back to healer and DPS. This quote especially is quite interesting;
Given the structure of the new talent trees, would it be possible for Blizzard to create a dual-role spec that could do either healing and damage based on talent selection? That certainly would match a spec based on the black dragonflight and a prismatic spec.This is a spellcaster class at its core. The class is initially only going to have two specializations, but if they do have a third, ranged dps and healer is where they’re focused.