Thanks to TrainerAsh0 for suggesting this Mega!
So Mega Ninetails would get the ability magic bounce, and gain the psychic type. Take note that with its hidden ability, drought, you can set up sunny weather before you mega evolve, making fire moves like flame thrower more powerful. Mega Ninetails also gets a large sp attack, sp defense, and speed boost. As for appearance, Mega Ninetails would get stronger and bigger. It would get a golden hue around it. Its tails get red on the tips, and its eyes become deep red.
Stat boosts:
35 sp attack
35 sp defense
30 speed