Ok. Let's see what we got here. We know that the next chapter shows Zeref and Mavis correct? YAS!
Because of the last page of 448. But isn't it funny that their is a quote saying "The truth behind the single magic....is it "love" or "darkness".......?" Then after that is an "image box" (I don't know what's it called) showing Lucy Heartfilla......Now is when serious shit starts going on. Sorry for my language but it's gonna go down from here. Holy fudge. OK LETS START ON MY THEORY.
Lucy Heartfillia stated in the manga that her mother taught her that One magic is actually "Love". So after Mavis announced that she was going to talk about one magic it must have trigger maybe a memory that she use to have with her mother. Seeing why she was the last character to be shown on the last image box of chapter 448. There was no need to have Lucy Heartfilla be the last character shown but she was. Why was it? Because she was the main character? Perhaps. Or was it something else. It was her mother Layla Heartfillia. The author is hinting that Lucy has something to do with One magic. As he as done so with giving out tons of hints of how END is actually Natsu Dragneel. If you had read Rave Master her counterpart can be seen in its female protagonist. Ellie. Yes. They even have the same loving pet. The snowman shivering dog we all know. Plue! Moving on, Ellie had a power so vast, that many tried to capture her and use it for their own purpose. It was Ethernion. But what about Lucy, she seems plain. Just a regular celestial spirit Mage with a sad past. Normal right? But I find it peculiar. She has to have some sort of power right! She must. And that leads me to her mother. The lovable mother that we all know but her background is so damn vague! Layla Heartfillia. Now this is my theory, listen closely, or read, anything would do.
Layla Heartfillia knows something about One magic. She clearly truly does know about. But that ultimately lead to her death. The reason being that she was cursed and why she died a young age after giving birth to Lucy. Mavis and Zeref were also cursed with something trying to find what One Magic really is. Leading me of how Layla was cursed as well. She even told Lucy about one magic too, but maybe she was too young to understand just yet. That explains why in the manga of how Lucy thinks One Magic is love, and how this recent chapter hints that love is maybe One Magic. Now. Zeref and Mavis fell in love but were both cursed. They tried to find One Magic to help cure their curse so they can live a happy life. But weren't able to do so. Same thing with Layla. She was cursed, she tried to find one magic to live happily with her family. But weren't able to do so. She dies and Lucy gets a sad childhood. Funny thing Is, I find something is brewing and I know it.
Layla died the same day Dragons disappeared. She must have been the one to have sealed the dragons inside the dragon slayers. She knew she was dying because of this, so she used up all her magic to seal the dragons.
So Layla has something to do with One Magic, she must be. To pursue it to stop this incurable curse out of "Love". For her family. The "Love" for someone to pursue something so great.
So my theory to sum up, is how Layla has something to do with One Magic and the reasoning why she died. Layla Heartfilla will show up in the future chapters hopefully and we will finally know her vague background. And maybe what this incurable "disease" actually is.