I guess the question is at what point does it become a completely different game? Your modders presumably need to have a UE4 licence - if they want to release a modified product they’ll need to have a licence for the other middleware they’re using, like . Without a licence key they could still change everything using Blueprint, but distributing or selling their new game executable would be dicey. If they’re distributing data for your game, that would presumably be fine (IANAL).