How about apps, which can only be purchased, if the user owns a different app? For example: I own green lantern for 3,99.-, but the green lantern, the missing chronicles comes out for 1,99.-. It can only be purchased by users which own the green lantern app. The add-on should link to the base app, and the base app should link to the add-ons.
There can be a combined purchase for 4,99.- or something as well.
Why all this? Sometimes apps stop selling well, but have a fan base which would be interested in a major update, which otherwise wouldn't be worth the development time.