Ella put in her letter to Christie and Emily that Christie isn't physically compatible with becoming a superhuman and she hasn't found a monster that Emily is mentally compatible with, so she would end up coming back to recruit the MC when she needs him. I don't think she'll have a pill for Emily at any point in the story. I thought Emily being infected by Valraven, or even Valraven's progenitor would be cool. Shadow Knight Emily would be very fitting with her goth aesthetic.
The way Ella says is "While physically you are better suited than Mc or me, I'm afraid mentally you are a little too unstable. I also sadly don't know what the right match for you would be even if you could get yourself under control".
I don't think she means being "mentally compatible" with a monster, instead it seems that she is too mentally unstable to make it through the transformation, whitout monsterfying i guess.
Maybe Ella never met a monster she thinks is compatible for her but no matter the monster she still thinks she is too unstable. Thinking about it now i wonder if Emily can simply find a monster and be infected even whitout Ella's interference, i mean someone more compatible than Mc and Ella, the chosen of 3rd and 4th could very well be a good target even for a numbered.