With Dumbledore being gone, Voldemort no longer has anyone in which he fears.  Therefore, Voldemort finds a way to take over the school.

 

In the first chapter of book six, we will not only learn of what conspired the night in which Voldemort was destroyed, but we will also learn of the circumstances of Voldemort's birth.

 

In "Harry Potter and the Chamber of Secrets," when Dobby is in the Dursley's house, Harry asks if Voldemort has a brother and Dobby shakes his head.  This is probably a yes answer, who's his brother?  I guess we'll see.

 

Professor Dumbledore will be the Defense Against the Dark Arts teacher during sixth year.  Hinting furthermore at his eventual demise.

 

Lily Potter was a seer.  She knew that Voldemort was going to come after her family, therefore put some magic on Harry to protect him.

 

Dumbledore's pensieve will reveal information needed for what happened during Voldemort's reign.

 

The Hand of Glory will become important.

 

In book three we learn that James saved Professor Snape from death.  Therefore, causing Snape to have a life debt.  Snape goes against Voldemort and tells Dumbledore that he is planning to kill the Potters.  Therefore, gaining Dumbledore's trust.

 

Dumbledore won't let Snape be the Defense Against the Dark Arts teacher because he is afraid of Snape turning back into a Death Eater.

 

Harry Potter is a metamorphmagus.

 

There is some connection between Mrs. Norris and Professor McGonagall.