@skyfire1207 Suggested this. Thank you!
Mega flareon would get (made up ability) blazinate, making normal moves into fire ones. This would give last resort, trump card and return STAB. Flareon would remain a pure fire type. Its attack is already Powerful on its regular form, at a base of 130, but after mega evolving its attack would reach truly lethal levels. Its sp def, and sp attack would also increase. As for appearance change, flareon would get a longer tail and fire would blaze out if its back like a mane. It would also get claws.
Stat boosts:
60 attack
15 sp attack
25 sp defense