The primary issues with accepting external content submissions are legal. There would need to be some kind of legal agreement set in place to establish the rules (as well as potential damages if those rules are breached) for the use of that content that all parties agree to. This would protect the developer from issues like the content submitter later reconsidering and deciding to sue for partial ownership of the game that their work was used in or for an injunction to stop sales of that game altogether. If that game content was created using libraries, tools, assets, etc. from others, those other rights owners could also have a legal stake in any game created with that content. This is why writing credits and payouts in music get so complicated - sampling other music is similar in principle.
These legal issues are why any sort of fan content submission contest is accompanied by a hefty legal contract that all participants agree to, including granting a perpetual license to the publisher to use that content without needing to pay for it, or even granting full ownership of that content to the publisher outright. This is also why the rank and file developers like me are forbidden from even looking at external content submissions. If something like that inspires me and I use some of those ideas in future content, that’s potential grounds to sue. Any such lawsuit probably won’t be successful, but it’s still enough of a potential problem that default policy is “we never read unsolicited submissions”.
Beyond the legal hurdles, there’s also the normal issues of content functionality. Even if content gets submitted that we want to use, there’s no guarantee that it is compatible with our workflow. A texture might be the wrong dimensions or format, there might be certain assumptions made about a particular piece of data that don’t work in all game modes, the submission might overwrite an existing asset and we’d have to create a whole new asset instead of replacing an old one. There’s a fair amount of effort that may need to be expended in order to incorporate external work into our internal workflow, and that kind of overhead is often not trivial.
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