If you do (or do not) recall: Snape, after six books of speculation, was a double agent inside the Death Eaters to ensure Harry's safety, fueled by his love for Lily Potter. He died in the Battle of Hogwarts, but not before showing Harry all of his memories and revealing his true allegiance. Because Harry's son is named after him, Snape's legacy is sure to play a role in this play.
Diggory was Harry's competitor in the Goblet of Fire. In the final round, Peter Pettigrew killed him on Voldemort's orders. He was the first person, but certainly not the last, Harry saw die during the Dark Lord's campaign.
Lestrange, who was Sirius Black's cousin and one of Voldemort's top Death Eaters, was literally blasted into nothingness by Molly Weasley ("NOT MY DAUGHTER, YOU BITCH!"). Here's to hoping we see more of Molly Weasley's fiery side this time around.
Minor characters could play major roles
Padma and Parvati Patil had bit parts in the Harry Potter books, but they did provide a lot of laughs as Harry and Ron's dates to the Yule Ball. They fought alongside fellow Gryffindors in the Battle of Hogwarts and are known survivors. Also rumored to play a role in The Cursed Child is Professor McGonagall, who served as headmistress of Hogwarts after the Second Wizarding War. Rowling, however, revealed that McGonagall retired from the position before Harry's son Albus attended Hogwarts.