I don't know about sharing inside the engine.. but it might work the same way.Here's how I will do it, every user will have an account and each account except their models, they will have like 10-20MB's of space where they can share models once in a while.And the legal part... I will just make the legal terms when they accept the license sound something like this... If the user wishes to upload and share 3D models trough that option, all the models will be considered as a donation to the game and company itself, and all the legal users inside or belonging to the company or game will have access if the owner of the model approved the upload.In case the copyright infringement is broken and the user does not have proof of owning the intellectual property, the 3D model will be removed from the upload or from the content of the game.
And of course.. user banned a few days probably.But there might be other problems... if the accuser demands the identity of the user and wants to sue him.But of course you probably can have a rule in the license for protecting that as well.But if he does't know who to sue?Dunno... just make sure it is in such way that you are not at fault that the user pirated that model.Or you can just give the user's identity, cause some rules must be rules.. probably
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And the part where sharing inside the engine... dunno if the engine is multiplayer and like 20 developers can connect to a project and work in the same time in different places that would be great as well and supposedly allowed to share their 3D models somehow.