The more I think about it, the more writing a Ghostbusters soft reboot seems like a monumentally easy task, and that they're failing so spectacularly is a literal cry for help from Hollywood. All you need to do is start with the existing story and expand from there. What happened in the 30 years since Vigo? Where would everyone wind up? How could new characters fit into that world without starting from scratch with some "why the fuck should I care about these characters?" moments.
Start with a world where there have been no ghost sightings since Vigo. Spiritual turbulence was considered an isolated incident, unlikely to repeat. The firehouse becomes a small museum dedicated to the paranormal, Ray resumes his Occult Bookstore, Egon passes away during one of his weird experiments on himself, Venkman and Dana eventually split up for good.
Now it's 30 years later. Enter our new characters. Let's do a Jennifer Lawrence/Emma Stone type who turns out to be Katherine, the illegitimate daughter of Venkman and the blond he banged at the start of Ghostbusters, and whom Venkman refers to as "Kitten" much to her annoyance. She follows in her father's footsteps, earning doctorates in psychology and parapsychology, and struggles to find identity among her peers who mostly consider her legacy to be a laughing stock.
Her mentor, perhaps played by someone like Amy Adams, is an expert in quantum mechanics, and they work tirelessly together to prove that not only were the events of 30 years ago not isolated, but that multiple spiritual dimensions coexist and can be crossed at will.
The third Ghostbuster would be your woman of color, Aisha Taylor would be a good choice. She's the niece of Winston, and the most spiritually knowledgeable of the group as an Occult professor and religious iconography expert. Often snarky because everyone is always ignorant of the origins of everything.
The fourth you go gravitas. Let's bring in Helen Mirren as the widow of Egon, who not only possesses the only working versions of the original proton packs and traps, but also has in her possession some new tools that he was working on before he died. Every bit the eccentric scientist and engineer, she's been trying to finish his work since his passing.
So what's the plot? Shit starts happening again towards the end of 2015, leading up to a climatic event happening on February 14, 2016 as predicted on World of the Psychic in Ghostbusters 2 (if you recall the first person to predict the end of the world was also right as Vigo came on New Years of that year). So tie it into Valentines Day. A worshiper of Pan or Lupercus is trying to bring back the ritual of Lupercalia, committing pagan acts and rituals which begin to unleash spiritual energy in New York again. The women come together, update the equipment and begin testing their theories as they investigate. They conclude that the incidents seem focused on romantically involved people, culminating in the fight with the big bad cultist and demigod. The cultist turns out to be Oscar Barrett, son of Dana and Venkman, who parallels Katherine's struggle with identity and the absence of a father figure. He'd be a concert pan flutist just to throw in an early clue and a reference to his mother being a symphony violinist. Then all you need is a purely comedic sidekick character and a decent minor villain foil like Peck and you've got a pretty solid movie doing both fan service and opening the possibility for sequels.