Add the ability for the uploader of a submission (eg a drawing) to tag users in or involved with it. For example, "Drawn By", "Commissioned By", "Lineart by", "Colors by", "Features character", etc.
I've merged in discussion from another thread about "Collab Submissions", where the permissions required were getting pretty complicated to allow joint control. This way, it's just a tag that's applied to a submission.
Additional features needed would include the ability to stop people from being able to tag you on submissions, and an administrative tool to allow for abusive tagging to be flagged and reported to the moderation team. Also the ability for moderators to add/remove people tagged on an image -and rules against harassment/abuse of tagging people (see Facebook for more details :P).
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