It amuses me greatly that there's two versions of Janine and Louis, but not the actual Ghostbusters. A natural expansion would be the four guys in their cartoon civilian attire:
Venkman in his white sweater.
Winston in his black and white polo shirt
Ray in his green sweater vest
Egon in his suspenders and pink shirt
I'd love Janine in the blue version of the “office attire” outfit, and her Now Comics flight suit (Egon's blue/grey with orange trim) but I doubt it would happen.
Some GB relatives:
Charlie Venkman
Cyrus Spengler and/or his sister in law
Lois Stantz
Big Ed Zeddemore
(Probably wouldn't happen, except maybe Charlie)
Ghosts and other things:
The Master of Shadows (from “Slimer, Is That You?”)
The Umpire from “Night Game”
The Headless Motorcyclist
Hobanagarik (“Cold Cash and Hot Water”)
Robo-Buster
(I could easily name more, but I tried to pick the most visually memorable from better episodes)
And if I really really could have my way, Nathaniel Blaque and Astorath from the Now Comics run, but that won't happen. Oh, and Dr. Bassingame, maybe packed with Charlie and Hob as a set :-)