There will never be DLC. The game is complete. Everything is known. If you look at the image linked on this same page, you will see everything.
There are 37 total Skylanders. The 32 regular ones, 4 legendaries, and Dark Spyro.
There are four adventure packs. The two unreleased ones are called Empire of Ice, and Dragon's Peak. The items and figures for each are also known. They are explicitly listed in the official strategy guide.
There are currently 23 of the 32 Skylanders released, not counting legendaries or the Dark Spyro, neither of which are treated any differently in-game other than their skin, and give you no special benefit to your "collection". The in-game collection only shows the regular 32. Although, it appears the 3DS version has all 37 in its "collection".
surely they will exploit dlc after the ones on the disc are used up, i mean, its easy money