Whimsicott
Typing: Grass / Fairy -----> Grass / Fairy
Abilities: Prankster / Infiltrator / Chlorophyll -----> Trickster (moves whose only effect is lowering one or more of the opponent's stats have their effect doubled)
BST: 60/67/85/77/75/116 -----> 60/77/85/97/125/136
Moves: +Nasty Plot
Whimsicott was the original Prankster; he was the one everyone thought would make a big splash in the OU Metagame at the release of Black and White. However, as the metagame progressed, people began realizing that it wasn't as good as theorized; it eventually fell from OU, even losing its spot as favored Prankster to Sableye. X and Y granted it the new Fairy-type, but also introduced yet another Prankster--Klefki--to compete with it. This theoretical Mega Evolution seeks to differentiate Whimsicott from its competitors entirely by giving it a new, yet similarly flavored ability. Trickster (it even sounds like Prankster!) similarly focuses on status moves, Whimsicott's specialty. However, it in particular focuses on a sorely underused aspect of the game, stat
lowering--particularly the lowering of your opponent's stats. By doubling only stat drops on foes, Trickster makes it so that boosting Whimsicott's own stats is not a necessarily better option than lowering that of your opponents. Mega Whimsicott seeks to take advantage of this in a variety of ways.
Whimsicott has a multitude of options when it comes to stat-lowering, so there was little need to give it any more. Fake Tears drops the opponent's Special Defense 2 stages, while Captivate and Charm do the same for Special Attack and Attack respectively. Tickle drops the opponent's Attack and Defense one stage each; between these, Whimsicott is capable of lowering all 4 of the opponent's main stats. Trickster turns the first three drops into a whopping
4-stage drop, and the last into 2 2-stage drops. With its newly given Nasty Plot, Whimsicott can then use the turn neutered opponents use to switch out to boost its Special Attack, or alternatively use Cotton Guard to raise its lower defense stats. Its boosted speed helps it move first against most opponents, even without Prankster.
Now, the stat drops Whimsicott can induce seem rather overpowered. However, despite the vast number of possibilities Trickster opens up, Whimsicott cannot take advantage of all of them--actually, it can only take advantage of precisely 4. Four-Moveslot-Syndrome is a major problem for Mega Whimsicott; only having an Attack- or Special Attack- lowering status move leaves it wide open to attack on the opposite side. On the other hand, having one of each severely neuters Whimsicott's own damaging or surviving capabilities, as it would be forced to either give up a healing, boosting, or attacking move to do so. Defiant (Bisharp) and Unaware (Clefable) are everywhere in OU, both of which stop Whimsicott cold. Its loss of Prankster leaves it susceptible to priority, especially things like Scizor's Bullet Punch and Mega Pinsir's Flying-type Quick Attack. Without Nasty Plot, Mega Whimsicott is unable to deliver much damage to opponents, since neither of its offenses even break 100. Finally, without Cotton Guard, Whimsicott's Defense is rather middling, leaving it particularly open to physical attacks unless it packs Charm or Tickle.
Mega Whimsicott may have its shortcomings, but its unique ability enables it to play a role unlike any other current Pokemon. Four-Moveslot Syndrome prevents it from being sufficiently prepared for every threat it faces, but with careful planning, it can be a potent, dangerous, and, most certainly, annoying foe to face in battle.