Nah, great film, but leave it be.
I think we've established now that belated sequels of this nature just don't work.
Oh, I think it could definitely work. The humour in something like Toy Story or the upcoming Wreck It Ralph (which looks excellent) is very similar to the humour of that in Roger Rabbit.
In fact the first Toy Story film opened with a new Baby Herman cartoon as a supporting feature - unfortunately I don't think it's available on any of the Toy Story DVDs.
There's no reason Bob Hoskin's character has to be involved in the story and there's plenty of mileage left in the idea of Toon Town.