Well, one thing that's important to focus on here is what kind of "future" content the digital expansion has opened up for you. Granted, the story on Makeb is 10-15 hours as you said; but the level cap increases to 55, and opens up all of the elder-game stuff to you, now and in the future.
So, if you think of it in that regard, you've just purchased something for 10-20 dollars, depending if you sub or not, and you now have access to all pertinent high-level content for the rest of that expansions life cycle.
I know that I'll probably catch flak since most people don't feel that it's a very good deal, but I think it is.
As far as what you asked, I believe that the future model will be similar to what we've seen, depending on how the game performs in the future. I would love to see a large retail release for a bigger expansion, but I'm just not sure it's in the cards right now. However, I'd like to see this, and I think it's possible.