Here's a few more:
Mediocre Pokemon made OP by its Mega Stone. A double Fake Out that not only gets STAB but also hits Ghost types (before it mega evolves).
Scizor is good in doubles anyway, but the Mega form is more viable in doubles because:
1. you're not choiced & locked into a move (switching in doubles is much less desirable because you're switching into 2 moves, not 1)
2. you're not life orbed so you won't sap your own health
3. Mega-Scizor receives better defences meaning there's more of chance it'd survive in doubles (if you provide wide guard support to avoid things like Heat Wave)
4. you get an attack boost without having to set up (130 to 150 from mega evolving)
Unorthodox usage of Mega-Gallade: give it Wide Guard and/or Quick Guard. It has passable bulk and a base 165 attack meaning even with 252 HP/ 252 Speed, it'll still pack a punch.
Same use as Kangkaskhan, a STAB Fake Out with Scrappy, but it's faster and more frail. Kangkaskhan can tank if necessary, but Lopunny is much more suited to being purely offensive, so it can pair better with bulkier/ slower/ set up team mates.
Swift Swim is good with Drizzle/ Rain Dance, paired with base 150 attack.
Bit of a niche, but Lightningrod is pretty useful if you pair it with Gyarados.