Mismagius
Typing: Ghost -----> Ghost / Fairy
Abilities: Levitate -----> Etheriate (Increases the power of Normal-type moves by 30% and changes them to Ghost-type; Ghost variant of "Pixilate", "Aerilate", "Refrigerate")
BST: 60/60/60/105/105/105 -----> 60/60/70/155/125/125
EDIT: 60/80/80/125/125/125
Moves: + Moonblast, + Sing, + Hypnosis, + Hyper Voice*
(Relic Song was also considered, but ultimately not included because of its status as Meloetta's signature move; flavorwise, the move would have fit due to Mismagius's affinity with enchantments, and the fact that Relic Song was some sort of vocalization that had a chance of inducing sleep, so let me know if you think I should change this.)
* Mismagius can currently learn Hyper Voice through a tutor, but it theoretically wouldn't be compatible with Mismagius's new moves because of this, so I'm proposing it as a permanent movepool addition.
Unlike its pre-evolution Misdreavus, who absolutely terrorized Little Cup, Mismagius has not made nearly as much as an impact in the tier in which it resides. That's a great shame, because in my opinion, it is very well designed; an ethereal ghost-witch who specializes in enchantments, curses, and hexes. It is in fact this that prompts me to add Sing and Hypnosis to its movepool, more for flavor than for anything else (although I'm sure some would be able to find use for the latter). It is also this that justifies the addition of the Fairy-typing upon its Mega Evolution. Mismagius's dark mischievousness and affiliation with magic makes it a prime candidate for either the Fairy- or Psychic-type; the latter has 2 4x weaknesses, 2 immunities, and 2 resistances while the former only has 2 2x weaknesses, 3 immunities, and a double resistance. Additionally, while Ghost/Psychic will likely be explored in the near future in the introduction of Hoopa, Ghost/Fairy doesn't seem to be coming any time soon, so it was the type I chose for Mega-Mismagius. Naturally, the inclusion of a Fairy-typing begs the addition of a better Fairy-STAB other than Dazzling Gleam, so I also added Moonblast to Mismagius's movepool.
"Etheriate" was how i sought to make Mega Mismagius differ from other quick, strong, Ghost-type special attackers, namely Gengar. Though new moves can't be added, new abilities could; thus, Etheriate became my way of allowing Mismagius to circumvent the fact that the strongest Ghost-type move without some sort of drawback or condition was the 80-BP Shadow Ball. Mismagius was already capable of learning Hyper Voice through a Move Tutor, and it also has a connection with voice in general due to its theme of enchantment. Thus, Hyper Voice would give Mismagius a nice 117-BP Ghost-type move to use against its opponents, distinguishing it from its main competition Gengar.
(For those of you who were curious or didn't figure it out yet, "Etheriate" is from the word "ethereal", meaning "celestial" or "spiritual." Hence, "Etheriate" is to "make spiritual", fitting for an ability that turns Normal-type moves into Ghost-type ones. For the pedantic out there, the mispelling of "ethereal" in "Etheriate" by changing the 3rd "e" into an "i" has precedence in the change from "aerial" to "Aerilate", dropping an "a" in the process. For those of you who could care less how I chose to spell a fictional ability that isn't even real, carry on.)
HP and Attack stayed the same in this Mega Evolution, HP because it can't change and Attack because I wanted to focus on Mismagius's design choice of being an enchantress, and thus focus on her special side. I did throw in a 10-point boost to her Defense, though, just to soften blows on that side a tad. Larger boosts went to her Speed, Special Defense, and Special Attack, with 20-point boosts in the former two and a whopping 50-point boost in the latter. The Speed boost put her just on par with the likes of Weavile (exactly a point behind Talonflame, though), while the Special Defense boost matched her stat to Milotic's (5 less than Mega-Gyarados). The Special Attack boost of 50 points has occurred before in Mega Ampharos, so it has precedence. And though Etheriate + Hyper Voice may sound broken, remember that Mega Mismagius is holding Mismagite (Pokemon with "I's" in their names have problems in naming Mega Stones), and subsequently can't hold any other boosting items. Mega-Gardevior has Pixilate and Hyper Voice, as well as 5 more Special Attack points, and no one is claiming that it is broken.
Since I'm up for a challenge, I'd like to reserve Ledian and Sunflora.