Suggestions by @user_185375

Brian Hull

as Soos in Gravity Falls live action 5 years later

Cole Sprouse

as Robbie in Gravity Falls live action 5 years later

Woody Harrelson

as Gideon Gleeful in Gravity Falls Future (Not Animated)

James Earl Jones

as Sheriff Blubs in Gravity Falls(animation 3D)Pixar

Chris Pratt

as Thompson in Gravity Falls(animation 3D)Pixar

Michael Cera

as Tyler Cutebiker in Gravity Falls(animation 3D)Pixar

Danny DeVito

as Grenda in Gravity Falls(animation 3D)Pixar

Arnold Schwarzenegger

as Manly Dan in Gravity Falls(animation 3D)Pixar

Michael Cera

as Deputy Durland in Gravity Falls(animation 3D)Pixar

Keanu Reeves

as Craz in Gravity Falls (1990's)

Tommy Lee Jones

as Agent Trigger in Gravity Falls (1980's)

John Astin

as Old Man McGucket in Gravity Falls (1980's)

Mary-Kate Olsen

as Mabel Pines in Gravity Falls (1980's)