How about open credits for writing only?

You would classify them under categories that roughly correspond to the current credits. The categories would be writer, OMB (which we may want to call something else), screenwriter and creator.

Writer would be for original scripts.
OMB would be for authors of adapted works.
Screenwriter would be for writers who adapted a previous work into a screenplay
Creator would encompass characters, created by, and anything else where a person's work is being used in some way, but it isn't an adaptation of their story.

I think most users do want to track all writing credits and writing credits are generally suitably distinction from other kinds of credits, so the data overload and confusion problems of open credits don't really apply here. The categories seem relatively straightforward (though it may get dicey distinguish between OMB and creator for loose adaptations) so should cause minimal confusion and since we'd copy the actual credit, we wouldn't have to worry about losing data by throwing something into a category.