Recently something came to mind that I found interesting about Raven Tail. To be more precise, it is the character Obra. There are several odd things about him. He never speaks and pretty much every action he does involves the little black imp that is constantly around him. However, the thing I find most interesting is his body, which apparently is nothing but a hollow shell, as seen after his arrest during the GMG.
Therefore the "real" Obra is the little black imp. This creature returned to Zeref after the GMG were over. So I ask myself, does this mean there is/was a connection between Zeref and Raven Tail? Perhaps Zeref send Obra to Raven Tail. Both sides had some benefits from this, since Raven Tail gained a member with a useful magic and Zeref was able to watch the GMG. Besides, Zeref probably knew FT and Raven Tail would get into several fights and therefore Obra would be ideal to gauge FT's power.
Aside from sending Obra to Raven Tail, it's possible that Zeref also informed Ivan Dreyar of Lumen Histoire. Considering the fact that Zeref and Mavis know eachother, it's safe to assume Zeref at least knows of Lumen Histoire's existence, though perhaps not the details about it. I know Makarov and Mavis said that Ivan probably got the information from Purehito, but this doesn't make any sense. Purehito knew Lumen Histoire's location, what was written inside and everything else there is to it. Therefore, if Ivan got his information from Purehito he would already know where and how to find it and even what Lumen Histoire is all about.
Raven Tail eventually disbanded after the GMG, but perhaps some ex-members will join up with Tartarus or Zeref and return to confront FT once more.
In a nutshell: did Zeref have any connection with Raven Tail? Will the ex-members join Zeref? Just who or what is Obra? The answers will be revealed in the future, but have your say about it right here and right now!