Huh. After my recent discovery of the Tumblr post, I thought it'd be enough to entice greater interest in showing support for the show. Maybe it's high time I pulled up this list of voice actors involved in the show.
CHARLIE ADLER: Stok, at least one additional voice
JAMES ADOMIAN: Gluteus Maximus (Todd), at least one additional voice
AZIZ ANSARI: Westley the Watchdog
WILL ARNETT: Ryder
RENE AUBERJONOIS: Maurice the Moon
ANDY BEAN: The Black Cube, Orbble Wright
JEFF BENNETT: Harvax, Galactic Conjunction Announcer, Brainz, locksmith (from The Big Job), a few additional voices
BLAKE BERTRAND: Jamie
STEVE BLUM: General Outrage, at least one additional voice
DEE BRADLEY BAKER: Bitties
CLANCY BROWN: Badlands Dan
COREY BURTON: Flea Captain, King of Flendar, a few additional voices
BRIAN CALLEN: Gil
SABRINA CARPENTER: Melodie
FULLER CORDON: Wilmur Wright
H. MICHAEL CRONER: Obvious teen (cycloptic), Chad
ANDY DALY: Andy the Watchdog
EDDIE DEEZEN: Cartoon Peepers
BILL FAGERBAKKE: Prince Cashmere
OWEN FAUST: Huckleberry Knucklehead
KEITH FERGUSON (he sure has a LOT of voices): Lord Hater, various Watchdogs, Dom Bots, Celestial Being, the Troll, Killbot-86, Blarpee's cashier, Intergalactic Guru (I think), Destructor's father, plumber (from The Big Job), restaurant manager (from The Helper), Dr. Scrivellix (the dentist in The Fremergency Fronfract), Mandrake the Malfeasant, many additional voices
RICH FULCHER: Worm-like spirit/Queen Entozoa/Sourdough the Evil Sandwich
GREY GRIFFIN: Olive (first voice, unnamed in The Nice Guy, name revealed in The Fremergency Fronfract), a few additional voices
JENNIFER HALE: Princess Demurra, Emily Ripov (full name found in the concept art)
JON HAMM: Cartoon Lord Hater
G. HANNELIUS: Little Bits
ALEX HIRSCH: Old Man (the Grunkle Stan look-alike), Soosy Du (the dog resembling Soos)
JOHN HODGMAN: Lord of Illumination
TOM KENNY: Commander Peepers, various Watchdogs, King Bingleborp, balloon guy, Fleeblebort, Neon Knight of Nalpraxis (Jim), General McGuffin (I think), many additional voices
KEN MARINO: Phil
JAMES MARSDEN: Sir Brad Starlight
SANDY MARTIN: Dorothy
LYONS MATHIAS: David
PIOTR MICHAEL: Major Threat (a.k.a. Jeff), Ms. Myrtle
JACK MCBRAYER: Wander, Cartoon Wander, a few additional voices
EDIE MCCLURG: Trudi Traveler (and her many clones)
CRAIG MCCRACKEN: Destructor's sock puppet voice
KEVIN MCDONALD: King of Sherblorg 7
SETH MORRIS: Neckbeard
LARAINE NEWMAN: Gram
DARAN NORRIS: Mittens
KEVIN MICHAEL RICHARDSON: Papa Doom, Knucklehead (fist fighter bodyguard), Clipper, a few additional voices
SAM RIEGEL: Emperor Awesome, hitchhiker (from The Helper), Something the So-and-So, Cartoon Awesome, Admiral Admirable, additional voices
ROB RIGGLE: Bill
JASON RITTER: Skipper (the Dipper look-alike)
KRISTEN SCHAAL: Mavis (the Mabel look-alike)
DWIGHT SCHULTZ: Wild Card
AARON SPRINGER: A few voices (including the two Okeydokian directors from The Helper)
JUNE SQUIBB: Stella Starbella
TARA STRONG: Beeza, alien queen (from The Axe), a few additional voices
JOSH SUSSMAN: Weaselly teen (snake/worm-like)
FRED TATASCIORE (he also has a LOT of voices): Dominator's masked voice, Captain Tim (actual), Captain Tim (Arachnomorph), King Dracor, Destructor, Lost and Found Guy, Thrax, Mooplexian, Frederick, Kragthar, slug boss (from The Night Out), many additional voices (including various creatures)
EDDIE TRIGUEROS: A few additional voices
KARI WAHLGREN: Planet Janet, Princess Galactia, Princess Veronicron
NOËL WELLS: Lord Dominator, a few additional voices
APRIL WINCHELL: Sylvia, Cartoon Sylvia, rock lady, Marsha, Oink, Rongruffle, Carol (one of the centipede-like creatures in The Night), Michelle, additional voices
CONNOR WISE: Hank
FRANK WELKER: Mystery Kids Mysteries announcer
ASHLYN FAITH WILLIAMS: Dental hygienist (a dental assistant, if you will), Olive (second voice)
CALLUM WORTHY: Teen leader (rabbit-eared)
"WEIRD AL" YANKOVIC: Dr. Screwball Jones
That's a lot of talent for one show spanning two seasons right there. I might be missing a few, even if they're minor, which is why I threw in "additional voices." And in parts where you see the words "I think," I'm unable to verify the voices, since the credits never go into greater detail. I'd sure as heck be curious to know what voice actors will perform new characters in season three, if the higher-ups at Disney ever get a change of heart, that is.