Frankly, I'm not actually sure where a flat ride could even fit within Celestial Park. It's weirdly built to feel both empty and pushed to its' bounds. The only empty plots in it to my knowledge are either too tiny even for a Storm Force, or have been earmarked for other ideas like the Potter gift shop.
Maybe the only proper chance for actual Celestial Park flat rides will come with the inevitable pathway between Potter and Stardust towards the additional Portal lands. Should they choose to push those Portals back behind Stardust and Broom Coaster a bit, then there is a decent amount of space around there to work with.