In addition to a new Pokemon, they announce that after you beat the main campaign, you unlock a 2nd Game mode on the title screen. This mode is exclusive to each game
Pokemon X gets the mode "Sparkling Aqua" (name not final) - A 3DS version of Pokemon Sapphire
Pokemon Y gets the mode "Burning Magma" (name not final) - A 3DS Version of Pokemon Ruby
The modes can trade to and from X/Y (with just one cart, and within reason so say, after you beat the E4 in Aqua/Magma you can freely trade, before then you can only trade Pokemon of a certain level dependent on badges), and can trade with other carts too.
Edit: These would have the Battle Frontier and other Emerald Enhancements