My theory is that they will figure out "what" are the first love zombies. Are they really just illusions and fantasies of man? If so, what are their purpose when they are not really being seen by the person normally? Why is no one seeing them other than Tarou and his other family? Those who can see them have "special" first love zombies too in the sense that they have a mind of their own.
Hmm. I think that "first love zombies" are not really just "fantasies" from their first love but might actually be the manifestation of a man's heart. Remember, in previous chapters, First Love zombies can affect the person like when Eve hugs a first love zombie, the FLZ's owner gets courage to pursue their first love etc. This can be that when a person fell in love for the first time, a "piece" of his heart manifests itself in hopes to be given to a girl. When the girl accepts, that means that the "piece" is accepted and disappears into her. They didn't really disappear, they just reached the destination. This is why no FLZ becomes unhappy when the love is accepted. It's because that's their purpose.
This is also why they turn into demons when they are unaccepted. They lose their purpose and cannot reach their destination anymore, their only option is to either die down then (when their owner moves on from his first love).
The only reason why Eve and the other FLZ has a consciousness of their own is because their owner CAN see them. Being aware of their existence gives the first love zombie to be aware of its own. But with that case, Tarou is only falling in-love with his own heart (or his self for that matter?). Or to elaborate the metaphor, falling in-love with Eve is like when a person holds onto his fantasy of first love. Not giving the "love" to the supposed recipient, and therefore not really moving on with their life. He is only trapping his self.
So my guess for the ending is that Tarou will confess to Ibusuki (but not after MANY tribulations and hesitations and doubts). Maybe Ibusuki will accept or maybe not (if she became good friends with Eve, maybe she won't want her to go either). But in the end, when they realize the purpose of FLZ is to be accepted (they don't vanish, they remain in a person's heart where they belong), Ibusuki will accept it or maybe Tarou (in case it's Ibusuki who confesses).
Then years later, Eve will be reborn. Tarou and Ibusuki will have a girl that looks like her and they will name her Eve.