After watching the trailers there are a few possibilities which seem likely
1. Quiet is Sniper Wolf in her younger days. I actually think Sniper Wolf's death scene in MGS says a lot about what MGS5 will be about. Check it out here:
Her connection with Big Boss is made clear. Her being a Kurd, Big Boss being in Afghanistan matches up too. "The governments of the world turned a blind eye to our misery...But then he appeared ... My hero...Saladin...He took me away from all of that...I became a sniper..."
I think Big Boss will be forming his mercenary army here but this time with no ideals. Just dogs of war which he will charm with his charisma into thinking that their purpose is just and they are not dogs. But it will be just out of revenge on the world on make profit from war. He will justify it by saying that the young victims would not survive had he not given then a purpose, a raison de etre>
2. Eli the kid is, you guessed it.......Liquid Snake. He is described as a youth who curses his fate. His fate being his genes. Notice how much he looks like Kaz Miller who he impersonated later in MGS. Sounds like Liquid is a dead ringer for ELi to me. Liquid would have been nine at the time too.
"I'm just the leftovers of what they used to make you. Can you understand what it's like to know you are garbage since the day you were born?"
"But I'm the one father chose"
7+ mins in