Suggestions by @iamanerd

Hartley Sawyer

as George Jetson in Hanna-Barbera Cinematic Universe

MatPat

as Writer in Gravity Falls

Danny Elfman

as Composer in Gravity Falls

Bill Murray

as Major Monogram in Phineas and Ferb

Dee Bradley Baker

as Perry in Phineas and Ferb

Mark Wahlberg

as Doctor Doofenshmirts in Phineas and Ferb

Brenna D'Amico

as Isabella in Phineas and Ferb

Tom Holland

as Ferb in Phineas and Ferb

Jake Short

as Phineas in Phineas and Ferb

Alex Hirsch

as Bill Cipher in Gravity Falls

Finn Wolfhard

as Lil' Gideon in Gravity Falls

Tommy Lee Jones

as Stanford Pines in Gravity Falls

Robert De Niro

as Grunkle Stan in Gravity Falls

Dove Cameron

as Wendy in Gravity Falls

Bailee Madison

as Mabel Pines in Gravity Falls

Kevin James

as Soos in Gravity Falls

Gaten Matarazzo

as Dipper Pines in Gravity Falls

Zac Efron

as Gambit in Gambit(MCU)

Vinnie Vincent

as Lobo in Super

Joseph Gordon-Levitt

as Superman in Super