Sayingkingkilla
And if they leave out Dana and Louis, I'm really just going to get the feeling that the creators are just slapping things together because kids only really care about the Ghostbusters. They probably have no clue which ones Dana and Louis even are. Much like some of RGB, they leave out the supporting cast from the films because kids don't care about them.
Louis was in the cartoon for a while after GB2. He just kind of faded away because there wasn't much they could do with him. He wasn't a good Ghostbuster, and his actual job (lawyer and accountant) doesn't lend itself to a lot of humor that would be interesting to an eight year-old.
From a pure storytelling standpoint, because of his lack of Ghostbusting necessity or a compelling personal connection to one of the actual Ghostbusters, he's second only to Slimer in being the most expendable character in the franchise. RGB got along just fine without him for four seasons. But because Rick Moranis is an old friend of Ramis, and from what I've read is still enthused about the property, Louis will probably be in any GB3.
As for Dana, it would've been hard to come up with stories that didn't fall into either “Let's have her appear just to appear” or “Play Damsel In Distress AGAIN” And again, there isn't much else left (ie her romance with Venkman) that would've been interesting to eight year olds.
I don't totally agree with Dana's non-presence, especially after GB2 (though her not being in RGB before GB2 actually helped set up by implication the idea that Venkman and Dana had broken up) but I do understand it.