Why is Erin so irrational and incensed with Heiss? First of all, she was antagonized from a young age by non-believers just like him, and second of all, she just had one career wrecked over paranormal scandal, she'll be damned if she's gonna let that happen twice.
Near the end, Kevin says that he went to help before it was revealed that he just got a sandwich. The last time he tried to help the Ghostbusters, he became an Unwitting Instigator of Doom so perhaps he concluded that staying out of the way was the best thing he could do.
Rowan acts as a Foil for Erin. Both are people who believe in and actively seek ghosts, are friendless, and are ostracized for their beliefs. Erin is a college professor; Rowan is blue collar worker. When Erin's past involving ghosts is revealed, she tries to hide it and keep appearances while Rowan doesn't hide it at all and is viewed as creepy by everyone. Ultimately, Erin finds herself with thanks to The Power of Friendship while Rowan finds himself by turning into a ghost.
Early in the movie, Rowan comments to Patty, who is an MTA employee, that in the 4th cataclysm blue collared workers will be some of the last to be killed. Later in the movie, we see Rowan has worked many low level/labor intensive jobs. So, of course the blue collared employees will the last to be killed: Rowan himself is one and has a soft spot for people like himself. Possible a version of Even Evil Has Standards?
One aspect of the original Ghostbusters that always seemed a bit contrived is how, just when Ivo Shandor's psychic-battery building has gathered sufficient power to unleash Gozer, a couple of oddball scientists happen to achieve a breakthrough that'll allow their team to defeat that sort of menace. In the reboot, there's no such glaring coincidence of timing, because Abby's publication of her and Erin's book is what taught Rowan to conjure ghosts, in the first place, and the team itself was brought together by investigating on Rowan-induced paranormal activity.
After pranking Erin with the spectral recording fart joke, both Abby and Jillian laugh that, unlike her, no one even came over to listen. Chances are, this was because no one they'd previously met believed in spectral recordings to begin with.
So what exactly was that demonic looking ghost at the heavy metal concert? Well, given that it apparently has sensitive hearing, it was haunting a concert that was playing Ozzy Osbourne's music, and at the end Ozzy himself was "having another flashback", the implication seems to be that it's the ghost of a bat, specifically the bat that got its head bitten off at one of Ozzy's concerts!
Rowan's ghost enters the bathroom Abby's locked herself in through the sink's drain, rather than walking through the walls. Of course that's the first method this particular menace would think of to penetrate such a room: the scene of his boss yelling for him to come deal with a stopped-up something indicates he's the hotel's plumber as well as its janitor.
When mega-Rowan drags Abby along with him into the portal, why didn't he just squeeze her to death in his grasp while he had the chance? Because however much he'd wanted revenge, he knew that she'd most likely become a ghost if he killed her inside the vortex, and probably - thanks to the cataclysmic energies that were being funneled through them both, same as he'd channeled energy into his death to pump up his postmortem "muscles" - one even more charged-up with spiritual energy than himself. If he has to be banished to the Other Side, he doesn't want a mega-Ghostbuster ghost to be trapped there too, to kick the crap out of him for eternity.
Why does Rowan try to possess the Ghostbusters specifically? Any body would have worked, and they were a long way from the hotel. He might have been aiming for revenge, and destroying their proton packs made sense, but these didn't seem to be utmost priorities given that he zoomed off on the motorcycle at the first opportunity. He manifested at their lab because their equipment made it easier for him to cross over. In his original plan, his machine would have remained operational, and he would have manifested there, but Holtzmann shut it off, so he was forced to use the next best thing.
Slimer, like in all previous versions, proves that not all ghosts are about tormenting the living. Some just wanna go have a rip-roaring good time.
The fact that Kevin is able to procure a sandwich during the climax suggests that either he stole someone's abandoned sandwich, or there was someone doggedly manning a sandwich shop across the street from an apocalyptic hellportal. In New York, that's a plausible explanation.
Everyone assumes that the tour guide who first encountered Gertrude's ghost soiled himself, and that his denials are just him being defensive and embarrassed. It's possible that he's telling the truth, however; consider the last we see of him before the cut to the opening credits is him about to drenched in a massive pool of ectoplasm, a substance which would likely have the same soiling effects.
Holtzmann is clearly very interested in Erin during the film. She introduces herself to Erin with a pick-up line, gives her the lesbian salute (one that her actress always proudly uses), dances to "Rhythm of the Night" for her, and allows Erin to go first during a weapons assessment since she says she's had a long day. She also gives Erin her own personal Swiss army knife, obviously caring a lot about her safety. Holtzmann flirts with everyone, but it's clear that Erin is the main object of her affections.
Part of the way Rowan's ideas are dismissed before he gives them in the climax is when he's going around setting up the chargers. Whether to him or not, most of the people there are nice. At the rock concert, people seem genuinely interested in others having a good time. (people return his hellos, cheer, etc). In the Aldridge mansion the guide is at the very least pleasant (as he probably didn't believe in ghosts before his interaction with Gertrude), and while wary doesn't dismiss anyone. The caretaker of the mansion actively sought out Erin for help, and was kind to all three women. Both answered any questions as best as they could. (with the exception of the one about Ed Sr., which could just be nerves). And in the subway station Patty is very cheerful and the artist - while a jerk by painting on the wall - is clearly on good terms with Patty and doesn't dismiss them for asking about a ghost. By the time Rowan gives his big "People don't care" speech, it doesn't hold up. At that point the only real jerks are the deans, who are clearly just jerks by and of themselves.
One guest on Venkman's show in Ghostbusters II predicted that the world would end on February 14, 2016. That was the date when the first teaser trailer for the Continuity Reboot was released, effectively ending the original Ghostbusters world.
The car that Patty gets from her uncle which becomes Ecto 1 is a hearse. Hearses are cars that are used to transport the bodies of the deceased for funerals (usually from the church/funeral hall to the cemetery), so the car is still being used to transport the dead, now it's just transporting their spiritual substance rather than their physical substance.
Several of Holtzmann's sidearm gadgets don't seem like that big of a tactical improvement on the proton packs, and some have clear disadvantages (e.g. Abby's ghost-punching gauntlet requires close range; the grenades demand an accurate throw and are less precise). However, the new equipment is also a lot less prone to collateral property-damage than the standard particle beams, and one such item - Patty's "ghost-shredder" - actually appears to acccomplish something that the original continuity's GBs never managed to do: it destroys the demon-ghost outright, rather than just trapping or banishing it.
Then again, if you cross-reference to the IDW comics, the comics show why simply "destroying" ghosts is a bad idea in the original continuity - Ghosts obey the First Law of Thermodynamics. Ron Alexander created a compact Proton Gun that "destroys" ghosts by dispersing them into atomic mist, but all the psychokinetic energy is still present. This resulted in a scenario where multiple ghosts "destroyed" over a long period of time pulled their PKE back together to form one super-ghost that Death itself had to eliminate, Death being the "cleaner" of PKE generated by ghosts in the Ghostbusters comics, taking the energy to the other side.
Which if you think about it would make for a really good second reboot movie plot, all those ghosts they dispersed in the first movie by various means plus the local psycho weakening the walls between reality as we know it and... elsewhere, getting the attention of a destroyer level entity which gets set up way quicker then normal because of all the stray energy floating around.
Rowan could have just as easily been a Ghostbuster and a hero in this story, if not for his fanatical hatred of humanity.
Gertrude Aldridge's whole life and unlife brings a lot of Fridge Horror, despite very little exposition.
Not knowing what the heck set her off in the first place, she murdered probably five to ten people in one night, yet spared her father (probably out of love).
Then her father locks her in the cellar for the rest of her life, instead of risking the scandal or seeing his beloved daughter hanged.
And she WAS a beloved daughter, judging by the huge painting of her dominating the library.
The basement was very close to the center of the ground floor. So that means that Mr. Aldridge probably heard Gertrude scratching, knocking, pounding the door, and begging to be let out. For hours. Days. Years.
When Rowan's ley amplifier brings her back, what does she see? Some smug peasant MOCKING her pain and lingering demise with his sleight-of-hand trickery.
For the first time in a Ghostbusters flick, a ghost actually kills a human on-screen, and a living human commits suicide. Previously, deaths only occurred as backstory (e.g. the Scoleri brothers' execution or Vigo's Rasputinian Death) or via Inferred Holocaust. Granted, it's somewhat tongue-in-cheek because it's Bill Murray in his Remake Cameo, but it's still a line the previous films carefully refrained from crossing. If the Ghosts from Our Past: Both Literally and Figuratively: The Study of the Paranormal tie-in book is to be believed, however, Murray's character may survive, as he provides a foreword in which he reveals that, due to said encounter, he has had to pay a lot in out-of-pocket medical expenses and has renounced his former skepticism. Of course, in the next chapter, Erin and Abby casually mention that their foreword was apparently written by a ghost...
If Rowan is trying to bring about the "Fourth Cataclysm", then there must have been threepreviouscataclysms. That might be another Mythology Gag, as there were two previous large-scale "cross-rips" in the original two films, and another one in the 2009 video game.
Rowan's expression as he was falling through the portal was not one of fear or fury. More like annoyance. Since Rowan seems to know just as much (if not more) than the Ghostbusters about the spirit world, maybe the whole thing was just a setback, and he'll be back soon enough....
At the end of the film, Holtzmann thanks the group for giving her a family and a home and making her feel like she really belongs somewhere. Consider just how many reasons she might have had to be deprived of a stable family structure (female, queer, STEM career, possibly neurodivergent), and it makes it all the more saddening and also heartwarming that she's found what she's always wanted.
Given that Holtzmann is the main joke-teller of the group, she comes off as a Sad Clown.
The Sequel Hook: the Ghostbusters may have saved New York, but they've just drawn Gozer's attention to their dimension...