I'm pretty sure including Dana was more or less a next-to-the last minute choice in order to fully lock Bill Murray in on the project. He was very much against the idea of doing a sequel, moreso than the others. Everything could have been easily recycled (Janoscz, Hardermeyer, montage, going into business/going out of business, etc) and I sincerely doubt anyone would have actually noticed the similarities, but when you toss Dana coming to the guys for help, then it's going down an easily familiar road.
I think that within the scope of the movies, the relationship with Dana gives Peter at least some moments to shine on his own opposite the Ghostbusters (which is an odd thing to say, since Dana never appeared in RGB, yet Peter was given as much screen time and character development as the others, same deal with the comics, although Dana did appear in Legion and it was a healthy balance for a plot very unlike the movies). You take Dana out of the equation for Peter, and most of the film revolves around Ray and Egon and you'll just have Peter in the dust mugging for the camera. Ever notice that in the two movies, they get some of the better, action-oriented scenes as opposed to Peter playing the smooth pimp with Dana? He's not quite the after-thought that poor Winston is, but he's dangerously close to that level. And I don't bank on the fact that Harold Ramis would have developed Egon/Janine in the way the fans hoped to fill that void either...the only thing I'd like taken out of GB2 would be Louis and Janine's blossoming "relationship" (although I don't hate Louis as much as some people, and he did have a couple cool scenes for also being an afterthought).
I noticed this too in several of the game's demo footage that you've got Ray, Egon, Winston and the newbie...but no Peter. I know this is early, but I'm certain that Peter's role in the game will be minimal due to this reason.
I'm also only talking within the scope of the movies. It probably would have been just as easier getting another love interest for Peter to help drive the film, but I think he and Sigourney Weaver had a chemistry that only Andie McDowall could match when it came to Bill Murray and women (P.J. Soles is a favorite of mine, from Stripes, but she wasn't in the movie as much to drive the romantic aspect), so I think it probably would have come off lopsided if they'd even replaced Dana with someone else.
That said, I've always loved GB2.