I think we can all agree that the Cursed Child was....pretty bad but at the same time, it had some good qualities. But this is how I'd rewrite it.
First of all, the families would stay the same. Draco and Astoria, Ron and Hermione, Ginny and Harry. Second of all, Scorpius and Albus will remain besties.
Now on own to the storyline,
The idea of alternate dimension and time travel is awesome, so I'd keep that. But why would our boys, Scorps and Al do that? Well one possibilty could be that a DeathEater escaped from Azkaban, seeing that it was extremely easy to do so. The DeathEater is still loyal to Voldemort and he enlists his son Theodore Nott to help him. We know that all the time-turners in ENGLAND were destoryed, so Theodore decides to go to a different country and uses the Imperius Curse to steal a timeturner from there.
After that, he pulls a Barty Crouch Jr, and he decides to disguise himself as a Potions Master, Now our two boys are being besties, and accidently discover about it. Theodore threatens to kill them but Albus and Scorpius lie, saying that they want to help him. So Theo is all like 'sure' seeing that Scorpius is Draco's son. So after many attempts to bring back Voldemort, Theodore Nott manages to kill Harry Potter during the Battle of Hogwarts, and bring back Voldy at the same time killing Albus. And then I don't know what else to add....
