This is basically a cross-over idea for Tokyo Ghoul and Fairy Tail, taking place before the Edolas Arc of Fairy Tail in an alternate timeline. Anyone that wishes to take a crack at this, feel free to use whatever ideas are here - you don't need to use them all, and can alter or add whatever else you wish. This is more a guideline for ideas that just popped into my mind one day, not a script that somebody has to follow to the letter. That being said, I hope you at least enjoy the ideas I had, or that it inspires you to write something of your own. :)

Prolouge

The premise is that this would be an alternate timeline in which, instead of being sent to Edolas, said transfer never happens and instead brings other beings into Earthland - Ghouls, among which is Rize Kamishiro. However, Rize is severely injured by Rune Knights after they catch her "feeding," leaving her for dead. However, a Dark Guild member, who wishes to experiment with the "creature," takes her to the nearby mortuary claiming her to be a a murder victim the same mortuary where Lisanna Strauss's body is - and keeps her there on the pretense of wanting to await identification. The attending physician then discovers, to his shock, that both her and Lisanna are alive. The physician however is then killed by his assistant after the latter discovers this, as he is part of a dark guild and wishes to experiment Rize and Lisanna before Rize assumedly dies, using his magic to re-shape another girl's corpse to look like Lisanna and place it in the casket. Agreeing with the appraisal that Rize's injuries mean she will be more likely to die, he takes them back to his Guild and proceeds to operate on the two, transplanting some of her organs - namely her left lung, liver and part of her her unique ghoul organ, the kagune, which is a Rinkaku-type that provides high-regeneration and superior striking & cutting power - into Lisanna in order to save the other's life and restore her body, making her a hybrid of sorts.

But after leaving Rize for dead with the intent of continuing observation of Lisanna post-surgery, Rize ends up awaking due to her regeneration - incomplete due to her missing organs - and promptly eats the physician. The high concentration of magic in Earthland, which makes it's inhabitants stronger and more healthy, allows Rize to absorb some of Earthland's magic by eating one of it's natives, enhancing her already-supurb regeneration powers and restoring her missing organs with brand-new, freshly grown ones. She also discovers, much to her shock, that Lisanna receiving her organs has made the Take-Over Mage become a half-ghoul hybrid with only a single Kakugan (The state where a Ghoul's eyes turn red and the whites of their eyes turn black) developing in her left eye. In spite of being mildly repulsed and insulted that such a creature was created with her own organs, she admits that Lisanna "smells tantalizing" and tries to eat her, but this results in Lisanna awakening and going berserk, her own post-surgery scars healing at an abnormally high rate as her Take-Over Magic reacts to the Ghoul organs and blood in her system and increases her aptitude with them, allowing her powers to manifest almost instantly. Rize, shocked, is fought off as Lisanna runs off into the woods, but Rize doesn't give chase instantly, opting to devour the other Dark Guild members in order to satiate her appetite and restore her strength.

The next day - the second of the four prologue chapters - Rize tracks down her "prize," finding Lisanna lost in a small village trying to find passage back to Magnolia and the Fairy Tail guild, wishing to inform them of her survival. However, she is stymied by her inability to consume regular food, and subsequently horrified to find that she finds the scent of the people around her to be appetizing, drooling openly when the idea of devouring one of the nearby children pops into her head against her will. When Rize encounters her in public, the Ghoul assures Lisanna that she won't eat her here because she doesn't want to draw attention to them, denoting that they would both be killed as a result. Rize then explains to Lisanna the details of what she is, taking small amusement in Lisanna's aghast horror in response to her condition as a Ghoul and that she cannot digest anything besides human flesh. Lisanna vehemently rejects Rize's words, trying multiple times to ingest normal food but failing each time and regurgitating whatever she tries to eat, further amusing Rize.

Eventually, a disparaging Lisanna tries to run from Rize and the village - and from her condition - as Rize gives chase, sadistically asking Lisanna to make up her mind as to "give her a challenge" by unleashing her Ghoul abilities, or "make it easy on us both" and submit so that Rize can eat her. Confused and on the brink from what has happened to her, Lisanna tries to hide out in the woods and escape from Rize, eventually meeting other Ghouls and learning that, apparently, hundreds, if not thousands of them were brought into Earthland. One of the Ghouls that Lisanna befriends is Touka Kirishima.

In the third of the four prologue chapters, there is an overview of how, in the past month or so, Lisanna, befriending a few of the other Ghouls pulled into Earthland, tries to cope with her new condition, all but starving herself of human flesh and forcing herself to eat regular food, which gradually saps her strength. This eventually becomes an issue as Touka tries to convince Lisanna to eat human flesh in order to stay alive, but Lisanna rejects her repeatedly, and more vehemently each time. Eventually, it leads to a falling out between the friends in which Lisanna furiously states that she won't become any more of a monster then she already is. Hurt by her words, Touka believes that Lisanna considers all Ghouls monsters and would rather have nothing to do with them if she had the choice, saying that Lisanna, due to being a half-breed, doesn't even belong among the "monsters." Feeling turned and rejected by her only friend, Lisanna runs off, not coming back for several hours.

But soon enough, both friends regret their words and move to apologize - however, before they ever have the chance to make up, Rize crashes down on the group while hunting for Lisanna, even killing and devouring one of the Ghouls that Lisanna befriended. In a rage, Lisanna calls on some of her Ghoul abilities, finally eating someone's flesh by taking a bite out of Rize's arm in order to fuel herself - however, their fight is interrupted by another Ghoul; Yakumo Oomori, also known as "Jason," who has long pursued Rize with the desire to break her free-natured spirit, as he feels torturing someone like her to the breaking point would give him ultimate pleasure. He defeats Lisanna and escapes with her after she agrees to come with him alive to spare the injured Touka's life, much to her sorrow as Lisanna denotes that Touka was right about her and resignedly calls herself a monster, and a monster that even other monsters are disgusted by. She is then taken away by Oomori, leaving the despondent Touka sobbing in grief at her friend's fate and the self-disparaging conclusions Lisanna came too, which she blames on their fight.

Rize, by contrast, is infuriated that Oomori took her "prize," and while Rize tracks them, Oomori returns to the Dark Guild he has joined in his time on Earthland, created by many of the Ghouls that have been trapped here and yet are treated as monsters no differently then the world they came from. This Guild - Agori Tree - is where Lisanna is kept for ten days, tortured mercilessly by him. Lisanna becomes distorted from the torture, gaining a finger-cracking habit from Oomori and hallucinating images of Rize and of her Guild Mates, the latter of which vanish from her mind one by one as they cannot help her in her current situation and are distorted by Lisanna's feelings of self-loathing, imagining that all of them - even Natsu and her own siblings Mirajane and Elfman - would be disgusted with what she has become. Eventually, only Rize exists in her mind, slowly seducing her into letting lose her Ghoul side. But when she refuses to give from physical torment, Oomori forces her to pick between two people to save - both of which had come to befriend her during her captivity - only to kill them both when a distraught Lisanna cannot make a choice.

In the fourth of the four prologue chapters, the trauma of seeing two people that had befriended her die because of her indecision sends Lisanna into a despondent state, locked in battle with herself as she laments what to do and is confronted by Rize's image once more. After trying to deny the image, Rize then denotes that would have been no different if it was her siblings Mirajane and Elfman in place of those two Ghouls, and that she doesn't have the strength to do anything about it - as well as the fact that as long as she keeps her siblings and her guild in her mind, she will never be able to accept her power or become strong enough to survive, and must abandon her ties with them if she wants them to be safe.

Rize then fatally splinters Lisanna's will and asks why they never learned she survived, slowly convincing Lisanna that they didn't put in nearly enough effort to find out if the corpse at her funeral was genuine and that her ties are only binding her down. Rize then asks if Lisanna wants to live, and Lisanna, tortured by her memories and the images of her Guild abandoning her one by one out of disgust for her Ghoul nature due to starvation and insanity influencing her mind, states that she doesn't care anymore and only wants to live on. This finally forces Lisanna to openly embrace Rize's image and consume it, thus accepting herself as a Ghoul and as a monster, breaking free of her restraints and taking a bite out of Oomori's face in order to regain her strength. She then fights Oomori, using her awakened Kagune to utterly overpower his Kakuja, pinning each of his limbs with one of her Kagune's tentacles (almost a mirror of the Kanaki fight in the series) before she sheds tears of remorse at what she has become.

Unknown to either one though, Rize has broken into the Agori Tree looking for her "prize," only to be stunned and surprised to find Lisanna has embraced being a ghoul, and watches as Lisanna brutally defeats Oomori, taking the lacrimal-based recording of the image as something of a twisted memento and denoting that maybe Lisanna is something "interesting" after all. She soon after reveals herself to Lisanna after the latter has won, urging Lisanna to finish the job and to eat him. After some marked resistance and internal conflict, Lisanna finally breaks when Rize denotes that Lisanna has already gone too far for moderation to mean much anymore, and that if not now then she will give in again later.

Rize then descends upon Oomori and began to eat him herself, tempting Lisanna by drawing out her feeding, making Lisanna slowly lose control over her inhibitions as she shakily admits that she is hungry and wants to eat. Soon enough, Lisanna, despondent but acknowledging Rize's words, slowly gives into her Ghoul instincts and joins Rize in devouring Oomori as he cries for his mother to save him, with Lisanna herself crying throughout the event as Rize helps feed her Oomori's flesh and Kagune.

After the two are full from eating Oomori's entire body, Lisanna, dazed and exhausted from her fight and from finally eating "properly," collapses onto Rize, nestling her head into the older Ghoul's chest and deliriously referring to her as "mother," her mental state regressing from the dementia she had been under and with Oomori's pleas of "mom" making her think of her own parent. Rize, dumbstruck a moment, slowly starts to smile and pet Lisanna's hair as the half-Ghoul falls asleep on her, musing that one could akin Lisanna to a Cuckoo chick that was dumped into her care, thinking to herself that she wants to see whether this bird can learn to fly - and when she's satisfied her curiosity, she'll decide whether she still wants to eat her.