Thanks to NintenDaan1, we finally have footage of the Pokemon X & Y English demo available to play at Gamescom 2013. The demo mainly shows the player battling against various trainers and Pokemon, and eventually confronting the Professor and battling with Mewtwo and experiencing the new Mega Evolution mechanic.
Nintendo has also revealed that a demo (most likely going to be the same as this one) to be playable by those at the PAX Prime game event at Washington State Convention Center in Seattle, which will run from August 30th to September 2nd.
At the Pokemon Game Show in Japan, there was a small game scene shown near the end of a Pokemon video, showcasing a Lucario and Blaziken, standing in the middle of a street getting ready to fight. The main video (before this scene) was about a Pikachu was running across the various Nintendo game consoles throughout the years and stopping in front of two 3DS XLs with Pokemon X & Y title screens.
Just recently, Siliconera has revealed that TCPi acquired new trademarks for both Pokkén Fighters and Pokkén Tournament. This could possibly mean that the scene is from Pokkén Fighters.
Japanese Game Title Screens: