Make Podcast the (secondary and unwitting) villain. In fact, I legit think making Podcast in particular the villain is where Jason and Gil were going in an earlier draft, but Columbia had a heart attack at making an Asian character a villain, so they just reused Gozer. Sounds crazy, but hear me out. The fact that he's obsessed with ghosts and the GhostBusters to begin with is a dead giveaway. The fact that he does a menacing podcast where he talks about such is giveaway #2. Hell, you could even throw in a bit of extra pathos for Phoebe's story arc. Have their friendship start out just like it does in the actual movie, right up to the point where they discover the ghost trap and accidentally release the ghost, who turns out to be Wovoka. Who is Wovoka? The Native American prophet responsible for the
Ghost Dance (because Oklahoma, plus it'd be a different mythology).
Have the Ghost Dance be banned because it caused an unholy supernatural event in the 1800s instead of what happened IRL (maybe that caused Krakatoa in the Ghostbusters universe?). Have that be what Egon was predicting instead of Gozer's return. Phoebe, with Podcast (reluctantly) and Trevor's help, traps Wovoka again. The dead communicating with the living freaks Phoebe out something fierce, but Podcast? Totally into it. He's so into it, in fact, that he wants to expedite the process instead of stopping it, which of course, causes friction between him and Phoebe. Phoebe make him PROMISE that he isn't gonna do anything stupid, which Podcast reluctantly goes along with at first, but his curiosity gets the better of him. He releases Wovoka, who possesses Podcast, and does the ritual. Chaos erupts. Native American spirits run amok through Summerville, making a whole mess of everything. Phoebe and Trevor are confused at first, then it hits her: Podcast. This tears Phoebe up something fierce. Here's the first friend she's made in, like, ever, and he turns out to be just a little too curious for his own good. It's around this time that she calls Dan Aykroyd and asks him for help, cause she isn't sure she can "neutralize" Wovoka/Podcast without killing the latter. Cue Egon's massive ghost trap field.
Phoebe, Trevor, Lucky, (who, in my improved script, had their relationship fleshed out a bit more), the summer school teacher, and the Spenglers' mom (having found a new appreciation for Egon much like she does in the actual movie) head into the mine to lure Wovoka/Podcast back to the trap field. It goes swimmingly.....until the time comes for the actual trapping. Phoebe aims her proton pack at Wovoka/Podcast.....but she can't do it, knowing damn well that without more proton packs, she could very well kill the only friend she's ever known. Cue the 3 original Ghostbusters showing up and doing their thing. No Egon spirit in my version. Wovoka is defeated, Podcast is back to normal, Phoebe and the gang live happily ever after. The end.