User blog comment:IamJakuhoRaikoben/Chapter 477: Transport, Review./@comment-4933386-20160315023949/@comment-7418318-20160315103254

Jacob wanted Lucy there so he could torture her, instead of just threatening to kill her.

I seriously doubt the illusion theory. That seems more like wishful thinking. If Mashima wanted to keep Serena in the story, he could always say that Serena was still barely holding on to life, but either August managed to stabilize him, or August goes to capture Wendy and somehow make her save Serena (force, convincing, making him appear as an ally with an illusion - take your pick).