EDIT: see "sanity check" below. While this list might work as a feature list for a ground-up application, the sanity check takes a much more pragmatic approach.

An artpack is one of (but not both at the same time):

• a set of "links" (references) to other artpacks

1.    references works as "directories", ie. they are not flattened (avoids recursion problems)
2.    conceptually an image is simply an artpack with a single element; more on this later
3.    can not include references to the type below

• a "list" of set operations that can be performed on other artpacks (sets) and metadata (see below) to determine the contents

1.    can freely flatten the toplevel of the first variant of artpacks
2.    can copy, but not include (flattened), other artpacks of this type; more on this below
3.    can include unflattened artpacks of this type

that

• can have associated "metadata" - creation time, description, contributors, number of pieces - whatever (depends on what we end up using them for)
• (wishlist) can be sorted

1.    by metadata in the contained artpacks
2.    by "sort-order" info in the artpacks, allowing eg. manual ordering

• (whishlist and libraries / performance allow) are partially ordered (ie. partially ordered sets)
• (wishlist) can be cooperative, with per-user / per-usergroup permission (user groups could theoretically be implemented as "art
  packs" ;))
• (whishlist) can be published / private to a user

which would allow us to use them for

• user art packs (obviously)
• user art collections (same as above?)
• "official" art packs (restricted, pushed to page)
• automatically generated art packs - anything goes here, really - by users/projects, most favourites/most views, by licenses etc.
• "Archives" (entries containing multiple images/files)
• Project pages
• "tagging"
• ...medal handling? Ok, ok, I know :p.
• More? I'd say as much as we want to, but we need to find a balance.

I might have pushed it a little, but this is the general idea anyway. Art packs are sets of (art-pack) references. "Submission/Entries" are ideally also "just an artpack". Scary huh? Anyway, this needs some clarification, but it will have to wait until I know what needs clarification, so ask away ^^. Keep in mind that this doesn't really take the Drupal/Database reality into account - it will need some sanity-checks.