Areelu Vorlesh has considered a lot of options for how to graft together the pieces of soul she has.  She's as certain as she can be that the pieces lack the integrity to integrate on their own, so she has settled on the strategy of integrating them into a host subject, with a desired end goal of substantially overwriting the host's personality and mind with that of the soul. It just leaves the question of exactly how.

She's considered a lot of approaches over the years.  She could take a gnome near bleaching and exploit the fragility of their existence, their hunger for new experiences, and insinuate the pieces in.  The gnome would regain their color and their personality would reorient to match that of the soul.  But the gnome would likely retain their original mercuriality and the resulting personality might not be stably retained. She could take an abyssal tiefling, and utilize the existing connection to the abyss together with the contamination of the pieces of the soul to join them.  But this bears a risk of excessive corruption by the abyss.  She could find someone that already matched the soul closely in personality.  This approach might be the most promising, but some trace of scruples or morality hold her back at the thought of doing that to someone so similar to the soul.

And she has the additional constraint that she wants to make them strong, strong enough that nothing ever threatens them again.  Her most potent technique for doing so would depend on the subject simultaneously integrating the power of the Nahyndrian crystals without being overwhelmed by them.  If the subject had an existing source of mythic power, that should counteract and stabilize some of the influence of the Nahyndrian crystals yet leave them open to absorbing the power.

So she needs a subject with an existing connection to some mythic source of power, yet somehow weak enough that she can capture and operate on them.  A subject simultaneously resistant to mental corruption, to withstand the Nahyndrian crystals, yet vulnerable enough to mental corruption that the personality and mind of the soul pieces will eventually predominate.  Ideally, the subject would already be powerful, yet not in the conventional way that would inhibit growing into more power.  And if she's willing to make the search for a subject really hard on herself to make everything perfect, the subject should be a human woman, her research into Druid magic suggested the dysphoria of a mismatch between soul and body can be quite uncomfortable, and she doesn't want that to happen as the soul predominates the host subject.

A lesser Archmage might give up or settle for only one or two of those criteria.  But in the course of her other research, Areelu Vorlesh has developed a workable wish wording for summoning a mortal creature that meets very stringent criteria.

She has her magic prepared so that the subject doesn't have even half a moment to resist.  It takes many complex spiritual surgeries and infusions before she thinks her work is stable.  At which point it is just the matter of finding the ideal conditions for the subject to grow stronger even as their mind is overwritten.