The most interesting is what they didn't show us,Anna's face. Considering manga hair is either white or black,and her hair is "white",it's most likely blond. She is also a celestial mage.
I wouldn't be surprised if she is another heartfilia and looks just like Lucy and Layla. That would explain why Mashima went through the trouble of hiding her face. Plus,if Anna participated in this plan,she most likely knew both Igneel and Zeref. This chapter proves the link between the dragons and Heartfilia,at least with Zeref and Natsu as the middleman.
However,I imagine Igneel was the one who brought her along to help the plan and then she had her descendent position herself as an anchor to july 7 X777. A imortal like Zeref could have easily made such a situation,especially considering that Brandish implied that Layla was from Albareth,but that would mean the timeline was altered. This could be a callback to rave master. In one timeline,Natsu and co failed to time travel since there was no one to open the gate in the future. Zeref then approached Layla and informed her about it and she linked the past with X777. Natsu and co would then time travel. His memory would have been altered because of the change in the timeline,lol. I think this is too complex and full of bullshit;
Or maybe,considering Zeref and Igneel simply followed Anna's plan,she kept them on the dark about the details and Layla actually knew about it. That would require Anna to have foresight,lol,and Layla to somehow know about the plan beforehand. That could easily explain why Layla worked for Zeref,considering what Brandish implied.
The timeline was still altered since timetravel is obviously not supposed to happen. Still,if Layla and Anna opened the doors,both of them must have owned the twelve keys of the zodiarc,so that would mean that it wasn't an accident that the twelve keys were separated from one another. It was intentional.
