Spielberg has been really close to Disney recently, so I doubt getting him on board would be much of an issue. I think the much bigger problem would just be licensing. Roger Rabbit had an impressive amount already, but many of its stars came from smaller companies with the major one just being Warner, granted Warner now owns most of the cameo characters. Nowadays, the rights to get cameos is probably expensive as shit depending on how far they go. Warner, Universal, and Viacom are the top dogs that Disney will have to contend with, not to mention whatever other smaller to mid range studio is around.
In the original Roger Rabbit, most of the famous toons came from Warner or Disney. Other than the occasional Betty Boop, I doubt many of the icons not tied to Warner costed that much. Nowadays, going for the main big animation icons since the 1990s probably costs a decent fee that will add up. SpongeBob, Shrek, Despicable Me, Batman TAS, Adventure Time, The Powerpuff Girls, Avatar The Last Airbender, Scooby doo, Madagascar, Kung Fu Panda, Ben 10, Total Drama, South Park, Rick & Morty, TMNT, The Loud House, Rugrats, Teen Titans, Fairly Odd Parents etc. are all mega franchises that would likely be expected to be there and they are all worth a small fortune in their own right, and that is not getting into anime if they want to go past western animation. In a way, this is prime time to attempt a sequel, but also, it may be far too costly given that animation is a franchised industry.