During a celebrity appearance, Lister revealed that there won't be just one Zootopia sequel, but two are already in the works...
“I can tell you for sure I’m doing another Zootopia with Disney. We doing three of them. [There were] three Madagascars. We’re gonna replace that. We’re the biggest film that Disney is producing. The last one was $240 million. This one I’m hearing will be $300 million. That’s what they spent on the budget, not the advertising. But the movie made $2.6 billion, with merchandising and the box office.”
Lister may be exaggerating those numbers, but they aren't really the important part. It makes logical sense that Disney would want to make a trilogy out of such a popular film, and it's not like there isn't enough material to work from. Having animals who behave like humans sets up plenty of opportunities to address real-world concerns in a fun and light-hearted way. Of course, all of this likely depends on how well a sequel does.