I have a pretty good feeling the Elder Elemental Eye (Who while a Greyhawk thing has intorloped in the FR before) will actually be fought. The end game opponents I am betting will be the Princes of Elemental Evil. Imix, Olhydra, Yan-C-Bin, and Orgemoch. These guys are setting neutral and have acully had a pretty good amount of buildup in the FR before 5e officially came out. During the playtest almost every adventure released then mentioned or had stuff related to them. Dragon and Dungeon before the hiatus had quite a few articles on elemental evil and the elemental princes towards the end as well.
This signifies to me that they wanted to do a Elemental Story line for a while now. They just wanted to start 5e with one of the standards of the game Dragons.