Changing the seasons:
Of course I'd have loved Prue back, but I'll make my changes to suite how the show was with restrictions and also budget in season 8.
I'd make Hecate, the Sources Queen of the Underworld, but dump the wedding from Hell episode. She'd still appear in human form, but assume the form of a Detective who'll work with Andy. Basically the season final plays out as normal, but we'd have Tempus working with Hecate, Andy stills dies. But she'd be banished to the underworld, and let slip that theres a higher ruler, but the charmed ones wont learn who it is.
The season final would introduce The Source in this instance briefly, so he'll have a presence during season 3.
What happened between season 3 and 4 is shown, how Phoebe got out of the underworld, since Shannen cant be brought back, footage is reused from the final showing Leo healing both Prue and Piper, only editing cut to Phoebe's reaction and a body double shown without Prue's face.
The Original Source is killed at the end of season 4, Piper learns shes pregnant at the end of the season as normal.
Basically the Cole/Source storyline until the 100th episode which would basically be Long Live The Queen. The Phoebe pregnant with baby Source would be dropped in this instance, given we'd already have Piper's pregnancy to deal with, so The Seer is replacing The Crone. Piper gives birth to Wyatt, but its The Seer who wants her child. The Seer would be vanquished at the end of the season.
Introduce Henry, so Paige/Henry dont feel rushed in season 8.
I'd actually keep Billie, I didn't mind a new witch in theory, but no Christy and ultimate power. Instead some plots from the comics, Prue would return but not played by Shannen Doherty, since they wouldn't be able to bring her back, so basically the comics story here. The big bad would be the first witch, the charmed ones would do another time-travel to the 1600's and meet Melinda Warren's child. No Phoebe/Coop, Phoebe does find love, but we do not see her love interest. Leo would not be frozen, but instead he would take Wyatt and Chris away with him for protection, so basically the final streach would focus on the core sisters, Leo would return last few episodes out of hiding. Darly would appear in a few episodes.