just an idea: Category Notification
i think that a category notification system would be very useful for vanilla forum users,
This means user can choose to be feeded about all the recent topics of a category (or some certain categories) that they chose.
For instance my forum contain lots of different category from gardening to cooking.
For example one of my user John, he is interested in gardening and want to share his experiences with others,through this addon, he can subscribe to the gardening category and even if he doesn't visit the forum, he can receive and notice the questions that he is capable to answer.
Since i have not code knowladge, i can not create it by myself, but if one of you - the developpers- make it, this will be very useful for the vanilla webmasters, and if you think to sell i will presumably buy it