What about Mega-Lucario?
Since Pokemon X and Y have not been released yet (coming Oct. 12, 2013), information about Mega-Lucario is not quite complete.
However, we first caught wind of this news on August 8, 2013 through CoroCoro, a Japanese video game magazine. The scan revealed the existence of Mega-Pokemon (showing that NewTwo is actually just the Mega-form of MewTwo), and revealed a little about how the mechanic worked.
The next day, Nintendo released a trailer, showing off the animations of the transformations (And they look pretty awesome!).
The New Form
Lucario "evolves" into his new form (only in battle, by the way) using a specific held item, called the Lucario-nite stone. This stone requires the trainer to have a special "device" equipped as well, and the trainer can use this to activate the Lucario-nite stone to get Lucario to Mega-Evolve. This mechanic also allows for Lucario to both evolve AND attack during the same turn, making this a very competitive option in comparison to popular held items such as the Life Orb.
One of the biggest changes to Lucario (Other than his art, of course), would be his ability in Pokemon X and Y (See below). From his sparsely used normal abilities, Inner Focus and Steadfast, Mega-Lucario's ability will be Adaptability, which, in short, raises STAB from dealing 1.5x damage to 2x, making Close Combat a wonderful competitive move and Aura Sphere an even-more awesome in-game move.
In X and Y
Play the game until you reach the Tower of Mastery. Once you get there, defeat Korrina, and she'll give you a Lucario, equipped with a Mega-Stone!
This new addition rejuvenates Lucario's activity in the Pokemon franchise and beyond, making him even more bad-ass!
In X and Y
The 448'th Pokemon will star in a game with two more generations of Pokemon that have come after him! Nintendo's/Gamefreak's actions have largely pointed to revitalizing older characters and connecting them to the new ones, apparent in this new season of anime and the Mega-forms of many older-gen Pokemon.
X and Y allow you to activate a Mega-stone and attack during the same turn; this is no different with Lucario. Upon activating a Lucarionite, his ability changes to Adaptability, which further increases the Same Type Attack Bonus (STAB) to 2X from 1.5X (For example, as of Generation VI, Aura Sphere has 80 base power; with Lucario, this power is amplified to 120, but with Mega-Lucario, this power is further increased to 160).
All of Lucario's base stats except for Special Defense are also boosted- Attack +35 (145), Defense +18 (88), Special Attack +35 (140), and Speed +20 (90)- making Lucario a quite competitive stone-holder in tournaments!
In Super Smash Bros. for 3DS and Wii U
Confirmed! Not only is Lucario in Super Smash Bros. 4, his Mega form is in there, too, as a Final Smash! Much like Wario's Final Smash in Super Smash Bros. Brawl, Lucario Mega-evolves into his new form in his Final Smash to do extra Aura damage. It's powerful but only lasts a short while, so it's not very strong compared to some of the other ones in the game.
In the fandom
GET HYPE! Nah, just kidding, but Mega-Lucario has received generally positive reception in the community, due to his formidable looks and his impressive improvements to his gameplay mechanics. Should make some interesting cosplay, too ;).