You don't really understand the concept of DLC, do you? DLC is bonus content that isn't in the game until downloading, you aren't missing anything technically. Everything that was meant to be in the game would be in from Day 1, DLC would just be an add on to extend the game's life.
I fully understand what DLC is thank you. In general, there are two ways DLC can be handled. The most common one that I find, which is the one you described, where the addition is installed directly upon receiving/purchasing the DLC. A second way would be that DLC acts like a key to unlock features already in the game (be it from the beginning, or added in an update to whatever game).
Either way, I'm not dismissing the possibility entirely, just that I don't find it likely that Gamefreak would make DLC to add features such as a battle frontier like area/musical/pokethon/whatever into the game. DLC that gives you access to areas with legendaries or rare pokemon sure, but not new features that all could have enjoyed from the beginning.
Would I enjoy full-fledged DLC? Absolutely. Its just, at this moment in time, I don't find it likely.