I'm both excited and a tad nervous that people are using the Assignment Bank theme for their own ideas, so thanks, Adam, for taking on the challenge... of challenges.
I have had some thoughts of something like an ability to create collections of bank items, that could be grouped, maybe sequenced, shared-- at one time I was thinking of playlists. At the same time, I don;t want to keep piling features on so it crumbles under its own weight and my spurious code capabilities.
So a cheap and easy way would be to use storify to connect the challenges you want to group together. Of course that rides on a third party service.
I can also see the way to build it within the bank theme-- a custom post type that can have a title, description, maybe its own category/tax taxonomy, and you somehow add the bank items (some kind of cart like thing? enter URLs? Select from a checkbox list?); all it would need to track is a list of item IDs in a post meta field.
I'm thinking of the bank myself for my new CC project, so have a reason to give the thing some attention. I have kept it away from users creating accounts, but that too might be an option to put in the mix.
Actually I have to fix the options panel still, the tabbed interface broke with a jQuery change. You can nudge me...