At this point, it’s been over 20 years since the original Who Framed Roger Rabbit, a film that perhaps single-handedly resurrected interest in the classic cartoon shorts of Hollywood’s Golden Age. Despite being wildly successful (thankfully, as it was the most expensive movie ever made at that point in time), a sequel was never made.
There are a couple of reasons why a sequel was never made. Number one, Hollywood wasn’t near as sequel crazy as it is today and number 2, the complexities of the original film made it somewhat more difficult to produce another one. No, I’m not talking about the animation, or anything technical, it is he sheer number of characters required lengthy and exhaustive negotiations between all the rights holders. You may have noticed that the original film had neither Felix the Cat or Tom & Jerry as the rights to neither were secured prior to production.
Why do I think a sequel won’t work? Well, why do you think a sequel wouldn’t work? Perhaps because sequels invariably share the same set of problems. TV Tropes has a good rundown of the symptons associated with what they call, sequelitis. The plot isn’t a continuation, bit characters that became popular are given way more screen time than they should be allocated, new characters appear that add nothing of value, etc. etc.
The original film was notable for many reasons, not least because it used a huge cast of already popular animated characters and introducing a few that appeared well known despite being brand new. That’s why Baby Herman, Jessica Rabbit and even Roger are still known, they have created a link in the audience’s mind between themselves and the classic characters of yore. A sequel will most likely copy some elements of the character but discard the deeper stuff that matters.
Another aspect is time. It’s been over two decades since the original and the times have changed. Roger Rabbit succeeded because it was different. Animation didn’t get a lot of respect from people in the 80s. Roger Rabbit (along with The Little Mermaid) helped change that and establish animation as an artform that could deliver the goods at the box office. There was little to no competition unlike today, where a new animated film is released, on average, every couple of weeks. The quality of said films is also astounding, thanks to the folks at Pixar who raised the bar so high.
Finally, as everyone knows, sequels inevitably have a lower budget than the original. In animation (moreso CGI than traditional) this is partly because computer models and sets have already been constructed, however, corner are still cut in areas such as story development, size of the crew, etc. The difference is always noticeable and in the case of Roger Rabbit, it would definitely be noticeable. If you make the most expensive movie ever and spend less for the sequel, it will look different.
Of course, there is the test film for a CGI version of Roger from 1998. It’s embedded below along with the test from the original film. The two cannot be compared in overall quality, but notice the difference in the animation. The newer one says a lot about the attitudes of executives towards sequels of classic films.