It is interesting I am seeing comments on how GB2 was trying to be dark. I never got that impression at all from the movie. If anything, I felt the first one was darker and this one was much lighter in comparison (likely driven by the popularity of the kids cartoon).
Yes, Slimer was added because of his popularity on the cartoon. His addition was a no-brainer given the character's popularity. Him helping Louis, I kinda dug. I took it as the logical progression of the minor Louis/Slimer subplot in that they've become friends as opposed to the general idea of Slimer helping the GBs (as he did in the cartoon).
Now, why Slimer was hanging around in the firehouse to begin with...well, that's another question all together.
I don't think it is worth trying to reconcile the cartoon and the movies. They don't work together and (beyond a random reference) never really intended to.
While it is not a bad idea at its core, I feel that's what'll do a potential sequel in. The chemistry of Murray-Aykroyd-Ramis was lightening in a bottle. Trying to replicate that with either unknowns or popular comic actors of today is a Herculean task rife with multiple inherit potential problems.
Well, the very name Slimer for that particular ghost comes from The Real Ghostbusters, the animated series, where he was the team's mascot. So I guess they were following that lead, although the sequel is hard to reconcile with the show's first couple of seasons.
Yes, Slimer was added because of his popularity on the cartoon. His addition was a no-brainer given the character's popularity. Him helping Louis, I kinda dug. I took it as the logical progression of the minor Louis/Slimer subplot in that they've become friends as opposed to the general idea of Slimer helping the GBs (as he did in the cartoon).
Now, why Slimer was hanging around in the firehouse to begin with...well, that's another question all together.

I don't think it is worth trying to reconcile the cartoon and the movies. They don't work together and (beyond a random reference) never really intended to.
I think that was the idea behind the third movie for at least part of the development process, to have the old guard handing over the reins to a new team, like the Extreme Ghostbusters cartoon.
While it is not a bad idea at its core, I feel that's what'll do a potential sequel in. The chemistry of Murray-Aykroyd-Ramis was lightening in a bottle. Trying to replicate that with either unknowns or popular comic actors of today is a Herculean task rife with multiple inherit potential problems.