Zola In Meredith's Shoes
Many fans believe Grey's might come full circle in the end. According to this theory, the show will end with Zola beginning the first day of her internship at Grey Sloan Memorial Hospital. The current chief of surgery would be Meredith Grey (Ellen Pompeo).
Some fans think that the ending of Grey's will reveal that everyone has been a figment of Meredith's Alzheimer's. Nothing and no one on the show was actually real, but Meredith sure thought they were.
On a lighter note, there's always the possibility that everything we've seen on the show is a representation of Meredith's writing. Like her mother, Meredith might have kept a journal all those years she spent at the hospital, and someone like Zola might have found it and shared it.
Everything Since Derek's Death Is A Dream
This one might throw you off a bit. Some Grey's fans believe that everything we've seen since Derek Shepherd (Patrick Dempsey) died has been dreamed by Meredith. Don't you agree that the events since then have been very bizarre?
Meredith Cures Alzheimer's
A big factor in Grey's Anatomy since the beginning has been Alzheimer's disease. Meredith's mother, Ellis Grey (Kate Burton) suffered from it severely leading up to her death. Derek even ran an Alzheimer's trial in hopes to finding a cure himself. And of course, there's always the fear that Meredith will develop the disease, being that it's hereditary. In the end, it would be interesting to see Meredith have found a cure!
A Tornado Hits
This fan theory is rather morbid, to say in the least. Some people believe that in the finale of Grey's, a massive tornado will hit the hospital, with everyone in it. All of the doctors will die in the tornado, and the show will end because no one will be left. This one is terrifying!
The Hospital Is Purgatory
This theory suggests that in the end, it will be revealed that Grey Sloan Memorial Hospital is nothing more than purgatory. All of the patients entering the hospital are already dead, and awaiting there final resting place. If the doctors are able to save their "life" they will then move onto heaven.