Well, according to Dave Gilbert on AGS, when you make an indie commercial game, an awful amount of time is actually spend pimping and plugging the games on every freeware, independant games or adventure community to get the word spread out, even the old games that are more than 1 year old. It seems to me like an unhealthy amount of time spend not creating.
One method that will be used by
Nifflas, for his next project will be to release Knytt Stories as freeware but charge to get the privilege to download several extensions and add-on levels from his website. It seems to me like a good compromise between offering something substancial for sale that will more likely interest the hardcore fans while not having to pimp his game on every websites out there, nor having to release an incomplete freeware game or only telling half of the story in it. While it wouldn't work for classic and linear adventure games, I suppose this method could be used if Yahtzee ever makes more games like GFW or 1213.
I do remember talking with someone who told me it was possible to create an add-on system for AGS, I suppose it could be implemented in games similar to GFW, and if we take GFW in example, could add new zones to explore with planets, enemies and subquests without affecting the rest of the game. As for games in episodic format like 1213, it seemed like Yahtzee had the game's mechanism finished and all that had to be done was changing the game's content, the method used for Knytt Stories would work here.
But only Yahtzee knows his games' sales numbers and could tell whether or not it would worth spending some sleepless nights to program such features or worth spending an entire month on bonus episodes that may or may not sell well.
By the way, if you haven't played Nifflas' games, I urge you to do so.