OK hear me out:
Mega Evolution: Mega Houdoom
New Ability: ~NO GUARD~
Reasoning: A hellhound spamming
Inferno not only looks and feels cool as
hell, but also gives houdoom a niche that doesn't exist anywere in Pokémon, making it viable and an actually good mega with insane anti-stall tatics.
Still managable by some special walls like Blissey, Heatran, Garga... shit movepool, and a not so fast speed tier (115 is good, but many mons already surpass that), still countered and checked by special greninja (unless sun is up), speed wake, M-Dragonite, M-Emboar and mainly Covert Cloak.
Sun makes it even better, but it comes at the cost of the best sun-setter that is also a mega