I'd say they could use DLC to make third/sequel games, but they'd make less money that way. Perhaps they could make it 20$ for previous owners and then 30$ for the actual cartridge like NSMBU: Luigi.
I see Pokémon DLC for main games being the same, but more advance, as it is now. We'll get event Pokémon through SpotPass (downloadable manually if you do so wish). Through the Global Link, we'll be able to customise stuff like the PSS interface etc. Perhaps get new clothes for our characters through promotions. All free of course.
Then, when the enhanced version/sequel/whatever they do next comes out, we'll get a free bit of DLC/patchwork that adds any new forms into X & Y.
I also hope that the Mythical Pokémon for the Kalos region aren't added until after they are announced, just through patches that are required to go online, and propagate when you wirelessly connect akin to the Berry Glitch fix in Ruby & Sapphire. This would make the reveals amazing once again. Last year, I was so sick of some of the responses when Nintendo revealed them. Things like "I have one already", "Show us a new generation" "I already know about these" and so forth.