Well obviously they need to get Dick Van Dyke, Sandy Duncan, Abott and Costello, the Harlem Globetrotters, and an archive recording of Don Knotts and Phyliss Diller.
Possibly Jabberjaw. He's voiced by Frank Welker anyway so its not like they'd actually have to bring in a guest.
Actually, do a whole season of Frank Welker guest stars as an inside joke. Garfield, DOctor Claw, Megatron, Ray Stanz and Slimer, the Smurfs, a pack of 800 different wild animals….
EDIT:
... there is a list out. Many of them were already in the traiiler.
-A few "I guess this is the level of celebrity they could afford" guests
-a couple "they probably actually wanted that person and have jokes and a plot based around their gimmick for them"
-one that is a very inspired choice, because they're doing it today and not 25 years ago. In the 90's is would have been super obvious, now it's just... what? I'm amazed the actor is willing to play that character again since they haven't in a couple decades and they swore to never play them again. Maybe they're okay with it just being the voice or it's just been long enough. Or they need money. Maybe they're Scooby fans.