In the background at the new facility, the Slime Blowers used in Ghostbusters II (1989) can be seen. One of the engineers is spraying another, perhaps to test the effects of the slime.
When Gary says he picked out a film for movie night, he holds up a VHS copy of Cannibal Girls (1973). This was the second feature film directed by Ivan Reitman who directed the first two "Ghostbusters" films. He's also listed as a producer on this film which is dedicated to him. The title "Cannibal Girls" can also be seen on a movie theater marquee in both Ghostbusters II (1989) and Ghostbusters: Afterlife (2021).
When Nadeem asks if anyone has a lighter, Ray replies that he stopped smoking in the 90s, whilst this may be a burden to their current predicament, it's a call back to Ray's bad habit, where he can be seen smoking throughout Ghostbusters (1984) and smoking a cigar in Ghostbusters II (1989).
Was released 40 years after the original film (and 35 years after the first sequel). As a bonus, it will come in time for the 100th anniversary of Columbia Pictures and the 10th anniversary of Harold Ramis' passing.
Ghostbusters: Frozen Empire is dedicated to the late Ivan Reitman, the original director of the first two movies who passed away on February 12, 2022.