Chandelure
Typing: Ghost/Fire -----> Ghost/Fire
Abilities: Flash Fire / Flame Body / Infiltrator -----> Entrap (Grip Claw in ability form; trapping moves will always last their maximum duration)
BST: 60/55/90/145/90/80 -----> 60/85/100/175/110/95
Moves: None
This one was fun! =3
Quick! When I say Chandelure, say the first thing that comes to your mind. If you said Shadow Tag, then you also probably know how hyped this Pokemon was when people first found out about its Hidden Ability in Gen V. You also probably know that for all intents and purposes in standard battling, Shadow Tag Chandelure doesn't exist, as GF changed it to Infiltrator in Gen VI. Perhaps this was for the better, since as we have seen with Mega Gengar, a Ghost-type with great Special Attack, nice Speed,
and Shadow Tag is a recipe for banishment to Ubers. But since Chandelure is so associated with trapping (at least in my mind), why not make a Mega that incorporated it in a way different from how Arena Trappers and Shadow Taggers do it now?
According to most sources, Chandelure hunts prey by surrounding them in a vortex of flames and burning away its victim's soul. As mortifying as that sounds, this is probably the basis of its Shadow Tag ability--it traps the foe and prevents its escape, just as the ability does. But Shadow Tag is something we want to avoid with Chandelure, especially since it is getting a 100-point boost to its stats as a Mega. How else could we make trapping, an integral part of both its flavor and its hype competitively last generation, a part of this Pokemon without resorting to the ability? Well, when one mentions a "vortex of flames", one particular move comes to mind: Fire Spin.
Semi-trapping moves are interesting in that they are in theory useful, and occasionally even see a rare bout of competitive play (see: Infestation). However, their traditional lack of reliabilty due to ranging from 2-5 turns has made them not very desirable in most matches, where even a single turn can decide the victor. Gen V helped by bumping the minimum number of turns up to 4, and while they are now occasionally still seen in the metagame, they certain aren't something that most people explicitly prepare for while teambuilding.
Entrap is a simple ability, both in terms of its effect and its coding (it's an item in ability form, like how Sturdy is for the Focus Sash and Aftermath is for the Jaboca Berry). Chandelure's main--and only--use for it is Fire Spin, a move that, though weak in power, is mainly intended to be used for its effect. With Entrap, Chandelure simulates Shadow Tag for 5 turns on the opponent, with the added bonus of the opponent losing 5/8ths of its HP plus the initial damage over the effect's duration. Now, you might say, "but Clap! 5 turns is more than enough for Mega Chandelure to completely wreck havoc on the opponent's team! Surely such an ability is going to destroy the entire metagame!" 5 turns would indeed be plenty for Chandelure to obliterate most teams with Shadow Tag; semi-trapping moves aren't quite the same as trapping, however (as the name implies), and carry a variety of limitations that help keep what would be a
very scary Pokemon in balance.
Firstly, Fire Spin has an accuracy of 85%; for reference, that is just a tad higher than the infamously shaky Stone Miss. Of course, relying on luck is by no means a way to keep a Pokemon in check, but it is something that makes it a bit unreliable, and not just a straightforward Pokemon. The key difference between Shadow Tag and semi-trapping, besides the turn limit, is the fact that a semi-trapping move won't take effect until
after the opponent switches, as switching always occurs before attacks (unless you're using Pursuit, which as no bearing on this anyways). This is
huge; what primarily made Shadow Tag so scary is that the user got to choose who gets trapped; it was ridiculously easy to simply switch in on set-up bait and proceed to, well, set-up and sweep. Pokemon were made entirely irrelevant simply because they were too easy to get set-up on by trappers. With semi-trapping moves, it is the
opponent that gets to choose who is trapped, rather than the user. This entirely changes how trapping is handled; Chandelure can't just switch in on set-up fodder to set up, since the opponent could switch into a counter to get trapped by Fire Spin instead, forcing Chandelure to switch and putting the user at a disadvantage rather than an advantage by giving the opponent a free turn. Instead of blindly and uncompetitively winning matches simply by switching on the right Pokemon, semi-trapping makes strategy a prime factor in determining how the game is played out. The opponent gets to choose who is trapped, for starters, so winning is not just a matter of setting up and sweeping. Instead, using Team Preview to gauge which Pokemon would be able to deal most effectively with the user's team becomes the main goal. The user then gets to switch in a Pokemon to deal with the one the opponent switched in--at this point, 2 turns of the trapping have been used up (one to use Fire Spin, the other to switch Chandelure to a counter for the Pokemon the opponent switched in). 3 turns of trapping is far more manageable for players than the indefinite number of turns Shadow Tag allowed. And of course, there are Pokemon that just outright ignore trapping altogether--primarily other Ghost-types, but also the Shed Shell user one sees now and then to avoid Magnezone and other trappers (as well as Flash Fire users in Chandelure's particular case). The one other notable thing Fire Spin does that Shadow Tag doesn't, however, is allow the trapper to switch; while it burns a turn of the trapping to do so, the ability to switch opens up a whole new avenue of strategies that become possible with semi-trapping.
I went very sparingly with the stat boosts, and gave it no moves at all, since Chandelure wasn't exactly weak to begin with and easy semi-trapping is a powerful thing to give to a Pokemon. For starters, I dumped 30 of the 100 points into Attack just to ensure this thing wouldn't get too out of hand. 175 Special Attack seems high, but considering that Chandelure has the second-highest Special Attack out of all non-legendary, non-Mega Pokemon, it would only be fitting if it had one of the higher Special Attacks as well. 100/110 defenses keeps Chandelure frail while preventing its Special Attack from going much higher, while 95 Speed is almost-fast-but-not-fast-enough-to-outspeed-base-100s (and rather slow for a Mega that wasn't already slow to begin with). Typing was fine, so I didn't see much reason to change it. The real star of Mega Chandelure is its ability, which gives it a unique niche that no other Pokemon in OU, even previous Megas from the submissions, have really capitalized on.
TL;DR: Chandelure continues its focus on trapping, but instead of capitalizing on the arguably broken Shadow Tag, it instead focuses on the unique semi-trapping abilities move like Fire Spin offer. The key thing that keeps it balanced is that the opponent can switch before being trapped, allowing him or her to choose who gets trapped rather than the user choosing as he or she does when using Shadow Tag or Arena Trap. A limited number of turns also prevents strategies like stalling and boosting from being viable with semi-trapping; however, the ability to switch and keep the opponent trapped also creates a variety of new options that no other Pokemon in OU can, making Mega Chandelure a unique choice for one's coveted Mega spot.
EDIT: I'm still iffy about the Special Attack. I tweaked it based on someone's comment, but I'm still not too sure about it. Some comments would definitely help.