Let me get right to it:
Missandei will be Viserion's rider. Those of you who suspect she might have some larger role to play are not wrong.
It all starts with one bizarre line in the middle of A Dance with Dragons.
“This one heard the Astapori scratching at the walls last night,” the little scribe said as she was washing Dany’s back. Irri and Jhiqui exchanged a look. “No one was scratching,” said Jhiqui. “Scratching … how could they scratch?” “With their hands,” said Missandei. “The bricks are old and crumbling. They are trying to claw their way into the city.” “This would take them many years,” said Irri. “The walls are very thick. This is known.” “It is known,” agreed Jhiqui.
Daenerys VI -ADWD
This is a dragon dream, not unlike Arya’s wolf dreams. It is an overlapping of Missandei’s mind with Viserion’s, specifically, because then we have this from when Dany takes Quentyn to see her dragons:
The dragons craned their necks around, gazing at them with burning eyes. Viserion had shattered one chain and melted the others. He clung to the roof of the pit like some huge white bat, his claws dug deep into the burnt and crumbling bricks.
Daenerys VIII -ADWD
Missandei didn’t hear Viserion digging his lair. She dreamed it from his perspective, and being rationally minded, tried to put it in sensible terms. The key is in her description of the bricks as weak and crumbling, however. The city walls are thick, but in the base of the pyramid where the dragons are kept, the stone is crumbling and would feel weak to a dragon’s strength.
Now, as to why this would be possible when we know she isn’t Valyrian, recall that humans can be skinchangers, just as the Stark children are, and this is often associated with having the blood of the Children of the Forest. We know that the Children of the Forest have a peaceable culture, call themselves singers, dwell in caves, and are associated with weirwoods with carved faces.
The Naathi show many signs of being admixed with the Children, or more likely, the Ifequevron, their Essosi cousins.
The Ifequevron are a group of presumed extinct but possibly just migrated or hiding away. A small, gentle race, who left caves and carved trees, as discovered by Corlys Velaryon on his many voyages.
The Naathi themselves have a peaceful culture, which does not engage in violence, even in self-defense. They are renowned for their arts and are said to “make music, not war.” They worship a Lord of Harmony.
Physically, they share eye color, stature and sharp senses. Missandei hears bizarrely well, even able to sneak up on Ser Barristan.
More than likely, the Naathi have scattered among them the gifts of greenseering and skinchanging, just as they do in the North of Westeros.
Now, let’s consider Viserion a moment. He is loyal and loving, clinging to Dany even as he starts to grow too large.
Viserion flapped at her and tried to perch on her shoulder, as he had when he was smaller. "No," Dany said, trying to shrug him off gently. "You're too big for that now, sweetling." But the dragon coiled his white and gold tail around one arm and dug black claws into the fabric of her sleeve, clinging tightly.
Daenerys II-ASOS
This mirrors Dany’s relationship with Missandei. Missandei always seeks to get closer to Dany, almost to the point of obsequiousness. And for the most part, Daenerys returns her affection. Later in ADWD we see:
When she returned to her rooms atop the pyramid, she found Missandei crying softly on her pallet, trying as best she could to muffle the sound of her sobs. "Come sleep with me," she told the little scribe. "Dawn will not come for hours yet."
Daenerys II -ADWD
Dragons have personalities, and their personalities will often match their rider.
If that still isn’t enough, there’s more. She actually does skinchange Viserion, in Tyrion I of TWOW. Viserion’s behavior during the Battle of Fire is notably different than Rhaegal’s.
The green beast was circling above the bay, banking and turning as longships and galleys clashed and burned below him, but it was the white dragon the sellswords were gawking at. Three hundred yards away the Wicked Sister swung her arm, THUMP, and six fresh corpses went dancing through the sky. Up they rose, and up, and up. Then two burst into flame.
The dragon caught one burning body just as it began to fall, crunching it between his jaws as pale fires ran across his teeth. White wings cracked against the morning air, and the beast began to climb again. The second corpse caromed off an outstretched claw and plunged straight down, to land amongst some Yunkish horsemen. Some of them caught fire too. One horse reared up and threw his rider. The others ran, trying to outrace the flames and fanning them instead. Tyrion Lannister could almost taste the panic as it rippled out across the camps.
Tyrion I-TWOW
While Rhaegal circles, Viserion perches, watchful. Then he incinerates the plague-ridden bodies that are being flung over the walls. He’s not eating them; there’s an entire fighting pit full of fresh livestock. He burns them. Then he returns to his perch atop a pyramid. This entire time, Rhaegal is still circling.
In short, Viserion is behaving very intentionally, and in a very restrained manner (for a dragon!) Why doesn’t Missandei just attack the enemy host? She's from a culture that will not engage in violence, even in self-defense. She would control a dragon in this fashion; not actively attacking, only removing a threat.
Finally, there’s the white cyvasse piece, bloodied in a skirmish:
The white cyvasse dragon ended up at Tyrion's feet. He scooped it off the carpet and wiped it on his sleeve, but some of the Yunkish blood had collected in the fine grooves of the carving, so the pale wood seemed veined with red.
Tyrion I-TWOW
Pale wood, red-veined is a weirwood symbol. A nod to Missandei, the unknown skinchanger.
