Tough call. I know it’s impossible to get all of the original voice cast back as they’re either too old or, in Earl Hammond and Peter Newman’s cases, dead. Getting Larry Kenny back though is a nice nod to both the old school fans and the original show; kind of like how it was getting Peter Cullen to voice Optimus Prime again in both the live action movies and Transformers: Prime, or Richard Hatch to play a recurring role in Ron Moore’s Battlestar Galactica.
We know Matt Mercer will be playing Tygra, and Kenney will be voicing Claudis, but the rest of the casting is still a mystery. Here’s just some wild speculation and/or wishful thinking on my part…
Panthro- Would like to hear Avery Brooks or Tony Todd. Second choices would be Keith David or Kevin Michael Richardson.
Slithe- I think Mark Hamill could provide an appropriately sleazy/slimy voice to this character. 
Grune- Hard to guess. Michael Ironside though is a great actor, and particularly good at playing heavies. He was phenomenal as Darkseid on both Superman: The Animated Series and Justice League. And I think his voice could lend the right amount of gravitas for the role.
Cheetara- Tara Strong is usually the go-to girl for characters like these. Not that I hate her. In fact, I like her. But I’d like to see someone else get a chance. If they can’t get her, they’ll probably either go with Grey DeLisle or Nika Futterman. A wild card could be Kari Wahlgren.
An actress whom I really would have liked to have seen in this role, though, would have been Ashley C. Williams, who starred in the independent Horror movie, The Human Centipede. And regardless of what anyone may have thought of that movie (I’ve seen it, and it really is as disturbing as they say and I would NOT want to see it again), she’s a phenomenal actress. She recently did a Fantasy musical you could take your kids to called Spellbound, where she displayed a wonderful singing voice.
WilyKat & Kit- I doubt they’ll get actual kids to do their voices. I think this time around they’ll go with a voice actress who’s skilled at doing kids’ voices. Probably Futterman, or maybe Candi Milo (she voiced Cheese on Foster’s Home for Imaginary Friends).
Mumm-Ra- It’ll be really tough filling Earl Hammond’s shoes. I honestly couldn’t even begin to guess who they could get for that one. Whoever it will be, I just hope he’ll be good.