Spielberg basically had to go around and personally negotiate with studio heads, if I remember the story correctly. If he wanted to do that again, he probably could, but I know I don't want Robert Zemeckis anywhere near that shit again.
If I remember correctly, the rumors surrounding a followup actually had cartoons interacting with video game characters? Something like that. There's been a bunch of rumors about it getting revived, but as you pointed out, rights issues make it kind of a no-go.
Besides which, 2D animation is at a low point in theatrical release, and watching people interact with 3D CGI isn't a novelty. It's 75% of all summer blockbuster fare for the past 10 years.
Yeah, even IF they someone got the money together to do it 2D and IF they got all the classics together again (which is pretty much the point of the movie - you can't just stick a Frozen Princess in there), there's no way Disney is going to allow all the smoking, drinking and (implied, I suppose) sex of the original in there.
It felt like I was watching a cartoon meant for adults as a kid, both in the way the noir world was presented and the age of most of the cartoon characters presented. It was the best.
Not that I'm saying I want there to be a sequel that somehow works mind you. Mostly saying that, for most cartoons involved in those earlier eras, they'll never get as cool as they were in Roger Rabbit again.
Frankly, for someone like me (who likes the 'golden age' of these franchises, and the sense of history that comes with it), I don't expect I'll ever get something new out of the Looney Tunes that's for me. Which is fine of course, I am almost 30 at this point. I can live with not having Bugs Bunny in my life.
I think there were a couple of 3D Looney Tunes shorts a couple of years back that had some good animation. Of course, it didn't bring anything new to the table, but I thought those were pretty well done. At this point, I don't think they can carry a full movie for anyone other than kids, unless it's going to be something crazy like Roger Rabbit and they go fully for the nostalgia of adults without worrying about selling toys and tv shows. Which isn't going to happen.