Just wanted to show what I think MegaGardevoir would look like. It would retain it's Psychic/Fairy Typing, and gain the ability Levitate.
The Red horn on it's chest would also split and come out of it's back, and become wings. You only see one because that's the side you see, but it has another on the other side.
Gardevoir (c) GameFreak and Nintendo, no copyright intended.