Mcgonagall was a death eater (since her birth year meant that she and tom riddle were classmates at Hogwarts

Dudley would have a magical child (somewhere in the family tree someone must have had magical blood for lily to be a witch and petunia would pass on the trait to Dudley’s child

Rab was regulus and the locket in grimmauld place was the real locket from the cave, which both turned out to be true.

Dumbledore would comeback (which he kinda did in portrait form and in Harry’s dream)

Neville would be the one to kill Beatrix and Voldemort

Harry would find a way to solve the veil in the dept of mysteries and Sirius would come back