If you forgot, Dr. William Bell was privy to all of Dr. Walter Bishop’s research which will probably lead to some conflict.
He will be ID’d in the finale, but that he might not be formally introduced in his first appearance, which could be sooner.
Casting spoilers:
Fringe is casting a sixtysomething actor in the role of William [...]

A NEW CHAPTER BEGINS WHEN “FRINGE” RETURNS AND AGENTS OBSERVE A MYSTERIOUS BOY AS A SERIAL KILLER RESURFACES
Just seconds before a building is demolished, a mysterious mute child that has been living alone underground is discovered. When the FBI’s Fringe Division investigates, the boy forms an unspoken connection with Agent Olivia Dunham. Meanwhile, [...]

On April 7 we definitely get an “Observer?” episode of Fringe. We say Observer with a question mark because the team finds a bald, silent and seemingly empathic child who never eats or pees and who looks a lot like an Observer mini-me, but whose, uh, peccadilloes could be explained by perfectly reasonable science. Then [...]

Mysterious William Bell
When this show finally returns from its endless hiatus, in April, we’ll learn a massive revelation about the mysterious William Bell. And we’ll find out more about how Walter, Olivia and Peter were all linked from the beginning.
Peter’s Past
Peter’s shady past will catch up with him, says producer Jeff Pinkner. In particular, the [...]

Here are the latest Fringe spoilers from Ausiello:
A main character will become pregnant by the end of the season. But there’s a twist. Guesses as to what it might be?
Spoilers sound cool, and male …

Are Peter and Olivia finally going to get it on?
Joshua Jackson sure wouldn’t mind it. “Peter is a shallow, vile, hedonistic, nihilistic man,” he tells us. “So, if Olivia would sleep with him, I think he would.” Now it’s just convincing Olivia to hop in the sack. That shouldn’t be too hard, right?
Olivia may have [...]

Fringe episode 15 titled as “Inner Child” , it airs on 4/7/2009. Here’s the trailer

Fringe Episode 13 Trailer called “The Transformation”.

According to creator Bob Orci, “Sooner or later the story of John Scott will play itself out, and maybe we get to a place where [Peter and Olivia] finally notice each other in a new way.” Scotty, you’re hot, but take a hike! [E!]