What would it take to achieve this?
I would have her free Mystra from the Staff of Savras that Cyric possesses.(That should be fun).
Mystra when she comes back gets her remaining Spellfire/Silverfire back from her remaining Chosen. Then as a reward Mystra 2.0/2.5 gives your player her divinity. Its a different take on how Midnight became Mystra 2.0/2.5. Then Mystra fades to black and reappears in Kelemvors realm.
Hope that helps.
Once this character has enough insight into the matter, she can turn towards the planes. Perhaps she might seek out the surviving faithful of Mystra, not necessarily Elminster or the like, but the few worshippers who still follow her despite her apparent demise, perhaps even just one or two allies. These people could provide the wizard with whatever information they themselves have accumulated over the past century. Perhaps they can even stoke the character's fire with rumors.
Maybe the tale of Mystryl's creation, born from the feud between Selune and Shar, may prompt the character to seek out the church of Selune, or Selune herself, for aid. Perhaps by reuniting the energies of both goddesses, the Weave may be reborn. The wizard communes with Selune, be it through a ritual, a mortal conduit, a vision/dream, or a direct experience with Selune, and, after convincing the goddess to lend her aid, receives an artifact which contains a shard of Selune's magic, the pure moonlight that helped to form Mystra. With this in hand, the wizard would then be led to obtain a fragment of Shar's energy. This might mean journeying into Shar's own lair in the hopes of stealing some sliver of darkness. Perhaps the energy may just emanate from the plane itself; perhaps it has to be in some concentrated form - the Karsestone is one such ancient artifact, that, over time, may have accumulated Shar's taint (it's only been in her personal possession for a century or so, but that's more than enough for her dark essence to rub off on the stone). The wizard may need to steal the Karsestone and then flee before Shar obliterates her and her companions. Alternatively, the wizard could travel to Sembia and brave the Shadowstorm; a century ago, Shar temporarily appeared between worlds as the Shadowstorm was being ushered into Toril. Maybe an imprint of her energy still exists in the heart of the storm. Once both Selune's and Shar's energies are obtained, the wizard may opt to travel to another plane in order to reunite the two properly; maybe there are "ruins" of Dweomerheart that still lie in the Astral Sea, which might serve as a proper spot for the rebirth of Mystra and the Weave. Once the deed is done, the Weave is reborn, and with it, Mystra; if the aim is to elevate the wizard to divinity, have the newly reformed Mystra absorb the character so that they become one - the new goddess of magic.
Alternatively... Perhaps Savras did not actually perish during the Spellplague. Although his visions were foggy prior to the murder of Mystra, Savras took this as a warning, and made preparations in case disaster arrived on his doorstep. During Cyric's attack and the annihilation of Dweomerheart, Savras managed to escape before he was destroyed; perhaps he had discussed this with Mystra, and they had both arranged for Savras to flee if there was trouble, so that her legacy may survive. However, during the escape, Savras was caught in the cosmological shockwave that surged through the planes, and became trapped, unable to escape or communicate with his mortal followers. A century later, a select few have begun to experience strange visions, of a deity with a third eye, or perhaps the open eye symbol of Savras. These people also gain the foresight and divinatory skills held by Savrans of old. The character learns of this, and meets with one of these people. Through her contact with this person, she is accidentally met with the same visions - she can see Savras bound, barely conscious, in some strange plane, the identity of which is up to you. Perhaps Savras is trapped in the Elemental Chaos, or had the misfortune of ending up in Baator or the Abyss when he tried to flee. Realizing Savras would have knowledge of the Weave, the character sets out to revive and liberate Savras, braving whichever plane holds him. Upon finally finding Savras, she realizes that the last spark needed to return him to full consciousness is the life of a powerful spellcaster - her own. Now, asking a player to sacrifice a characer's life is a tricky task; you need to convince her that her death will truly restore the Weave. So, between the initial contact with the mortal diviner and her encounter with Savras, have her experience a number of visions, which show both she and Savras interchangeably. The closer she approaches Savras during her journey, the more powerful the visions become; the more powerful ones display her in a way that surpasses mortal description (foreshadowing her ascent to divinity). This way, she should understand what's going to happen once she merges with Savras. Once she offers her life for Savras, he reawakens, and so too does the concentration of the Weave that was stored within him. With his return, the Weave comes to life once more, drawing on the pockets of Spellplague that still dot the land. The Weave grows, and it returns to the structured form it held before Mystra's death. Once this is done, the character is reborn from the Weave with the powers of a goddess. Perhaps she goes on to serve Savras in the age of the returned Weave, or maybe the roles are reversed. Perhaps, upon completing his task, Savras fades away, having performed that which he was fated to do.
Excellent advice.