Susan would see Amelia standing next to her. :( And probably her other dead relatives would be hanging around in the background.
Romilda would see herself at eighteen, tall and skinny and beautiful. Harry Potter would have his arm around her, and he'd be looking totally fit and manly. And she'd be wearing gorgeous clothes and perhaps be holding a BAFTA - or maybe even an Oscar! - because she'd totally be a successful film star by that age. And she'd model in her spare time. And maybe have her first album coming out soon.
(I'd be very impressed if the Mirror could convey all this...)
Draco would see himself standing next to the Dark Lord, his trusted right hand man, and all the other Death Eaters would fear and respect him.