A sequel/prequel only ruins the first movie if you let it. Too many viewers let it.IF it happened then, it probably wouldn't have been such a big deal, but after all this time, and the original being such the standalone classic and groundbreaking piece of cinematic history that it is, any sort of followup movie at this point in time would just tarnish the credibility of the original.
I have problems with long contested sequels, and how everyone wants one until they get it. I'm an open mind of seeing another Roger Rabbit story, and I have fan-ficcy ideas that will undoubtedly corrupt my opinion of the film if one happens. I still think there needs to be a direct sequel about the television age hurting the theatrical toons, and a big scale war breaks out between made of TV cartoons and theatricals with Roger getting in the middle (though I really hope Dreamworks would let them use the Classic media library if that happened). All and all, I think that Roger Rabbit 2 needs to bring back as many characters as possible, and renegotiate some of the characters that couldn't make it in the last film.
A sequel can be done properly, and even if this is a rumor, the one thing that is true is that any story needs to be filtered by Disney and Pixar. And we all know that Pixar is good at story telling. Even their weaker films are better than most of the CGI animated kiddy fare.