I was actually joking about how WB just uses Scooby-Doo as a lame crossover vehicle these days. Gravity Falls was just my first thought for a property. Also, Hirsch worked with Disney as late as 2023, so unless they did something to REALLY set him off in the past year I wouldn't call it impossible.

(just very, very unlikely after the mess the censors put him through during GF's production)