The concept is simple: a food boost potion effect. One that would counteract the hunger effect. The saturation effect exists, but it is problematic because it adjusts saturation, not hunger, which are two separate values.
I'd much rather see a potion that would be "Saturation Regeneration II". Very OP since that would make you very very durable with the rapid health regen that comes with it.
I think an end-game potion to replace all food would be neat, but your idea about hunger, and not saturation is better for balance's sake.