Hm... so about traveling to some other region after you've completed the game. I was thinking, instead of traveling to the Unova region from the region you start in, how about the airplane takes you to the Sinnoh region instead?
The Legendary Pokémon are also related to the dimensions theme, while I kind of think Xernias is about width and Yveltal is about height. Though we already know that Dialga is about Time, while Palkia is about Space. If the game is going to be about dimensions, then Dialga, Palkia and Giratina would totally match for Legendaries in Pokémon X and Y, and would be a perfect opportunity for those Sinnoh legendary Pokémon to make a comeback to be encountered in Pokémon X and Y, since we haven't seen them since Diamond/Pearl/Platinum.