So i have a theory and its like 70% wrong but i want to share it with you all. When the new opening arrived we see a zerefxmavis moment and i started thinking maybe they have a connection! Now i don't think Zeref accidently got to Tenrou Island maybe he did it on purpose because he knows Mavis. I think that in the past Zeref and Mavis were friends and she was the only one able to be control his magic. I think she created Fairy Tail with zeref and that he haves a fairy tail mark to. Zeref may killed mavis accidently when she lost control and wandered over the years at her grave. And a Arc that Zeref and mavis see each other again and start telling everything.
THIS IS MY BIG BANG THEORY =O