It takes design, testing and balancing time. For instance, you can't just shove a Chancey in the first patch of grass you encounter, cos it would unbalance the game pretty heavily. Similarly, you can't have an over-abundance of Pokemon in one area or a drought of a certain type of Pokemon in another (look at the fire-type in Pokemon Pearl as a good example of that). Another thing they often do is limit each route to having a half-dozen species of Pokemon, so when you start adding Pokemon you either have to add more areas, or work out some other way of fitting them into the game (like Swarm). So you're right in that it doesn't require any programming time, but it does require design time and some thinking, and that sort of stuff is very difficult to get right the first time you balance it. On that note, I would say that there's a really good chance of Gamefreak doing DLC. They've already done exclusive Pokemon in a bunch of different ways: we've gotten exclusive Pokemon from events (physical and digital), exclusive Pokemon from the Dreamworld, and if you look at the whole "each generation of Pokemon is split into two-or-more versions" thing it's basically the same principle. Now what I HOPE they don't do is make DLC the only way to get certain species of Pokemon. I would think that you'd find Pokemon with their hidden abilities, Surfing Pikachus, old legendaries, shinies, that sort of thing. Stuff that is difficult for people to get but not game-breaking.