All right, so bear with me.
Alternate universe where Perry is the human scientist, and Doof is the animal agent.
But Doof is so embarrassingly incompetent that Perry starts purposely making his schemes easy to thwart out of pity.
And Doof is Vanessa’s pet heron (“Curse you, Heinz the heron!!”).
And Perry is Candace and Phineas’ dad, and Ferb is a robot brother he made for Phineas.