Yeah, just a scene or two is good. I agree that having him around too much could hurt the film. Just have him show up for a joke or two like GB2. Wouldn't want him hovering around during a serious scene talking in gibberish like the cartoon. Though I certainly don't think his inclusion in GB2 hurt the film. Him driving the bus is one of my favorite scenes. Never understood the negativity towards GB2. Sure it wasn't as strong as the first film but it's not like it's terrible or anything.
Glad that others are hoping for a Slimer appearance. I recall reading an interview with some writer who was writing a Ghostbusters mini series comic and he made a comment about how he hated Slimer and would not be using him at all. If you hate Slimer then you're not qualified to write a Ghostbusters story in my opinion.