And he's still busy with his film and TV work, particularly the latter in the Roseanne reboot on ABC. So there was no way that John and Billy would reprise their roles as Mike and Sulley.
Looking forward to hearing more of Carlos voicing Mike as he does a good job voicing Mike in the video games when Billy Crystal is unavailable. I have yet to hear what Christopher Swindle will sound like as Sulley.
Pretty sure the Original cast for Frozen will return, especially Josh Gad for Olaf. Idina Menzel as Elsa goes, either way, she was able to return to voice her in the Disney INFINITY games and other merchandising, although in one of the Disney app games (don't know which one) Elsa was voiced by someone else.
Personally, it will depend on Idina's current schedule cause even though she doesn't have a tour until the Fall, it will largely depend if Disney/Square Enix ask her first and she's available to do it, but I'm certain she will come back.