I haven't seen anything that takes this idea into concept about Yanmega, so here it goes.
Yanmega
Name: ScarfMega
Bug Buzz
Air Slash
Hidden Power [Ice]/Hidden Power [Ground]
Hypnosis
Item: Choice Scarf
Ability: Tinted Lens
Nature: Modest
EVs: 0/0/0/252/6/252 (or the 6 in Def, your choice.)
We all know Yanmega has Speed Boost to become a threat that can not be outsped after a turn or two, but sometimes, Protecting on a turn to save yourself gives the opponent a free moment to set up. Yanmega's speed is nice enough to run a Choice Scarf, and it's a very nasty surprise to the opponent if they can't see it coming. It also makes a wonderful lead, outspeeding and quickly hitting Stealth Rock setters. It can also put to sleep leads that aren't running Lum Berry.
The EVs given give Yanmega 433 speed, which outspeeds a hefty amount of things, including some versions of Ninjask, giving them the assassination if they don't Protect (should they try Sub, they're dead unless they have more than 433 speed, which isn't too common.) Should they use Weavile, it's going to die, thinking it can Swords Dance on an anticipated Protect for the Speed Boost.
Tinted Lens is a very powerful ability, turning what would be something as a 3HKO back to a 2HKO. The choice of Hidden Power is whether you want to be more successful as a revenge killer or hitting Magnezone and Heatran, two common counters to Yanmega, much harder. Hidden Power Ice delivers revenges to Dragons, particularly Garchomp, locked into Outrage (Not Scarf versions though.)