For a use case of this idea, please refer to this forum post.
In summary, currently you can only access a single item in a list when using the adjacent features functionality. We should have the ability to access an entire list attribute, regardless of size/count, in order to copy it over to another existing or new list attribute. It would also be nice if perhaps the adjacent feature functionality is made available in other transformers such as the ListCopier, so we can copy an adjacent feature list attribute for each feature. Thanks.