I've been browsing online a lot trying to get info about the game but not too much about the storyline because I still want to be surprised next week.
I've seen photos online of the game showing Slimer in the Sedgewick hotel, in the firehouse inside a glass case that I assum is a smaller containment unit and even in the basement in front of the upgraded containment unit.
I wasn't sure if they are making him have more of a role like "The Real Ghostbusters" where he's like a pet or family member. His appearance in GB2 made me think he has an attachment to the team even though he looked horrible and overweight! :-)