User blog comment:BlackWizardZeref/Fairy Tail Chapter 394: Juvia vs Keith Review/@comment-25244540-20140725085202/@comment-25244540-20140725093207

I'd say Natsu's powers are more likely to awaken END than for him to become his vessel. Fire gets stronger with more fire, and Igneel couldn't beat END so maybe he used his powers to make a seal? If he did since Natsu's magic was taught to him by Igneel that means Natsu unknowingly could break that seal.

Since Yukino has Pisces and Libra's keys and the only Sabertooth members who've shown up are Sting and Rogue I don't think that's the case. Even if it was we have no idea what happened to Aquarius' key, and if it just got rebuilt somewhere else Yukino would have to find it.