That’s a different legal problem as there are situations where if you don’t protect your trademark you can lose it. But I’m not a lawyer and don’t know if that situation would apply to mods.
Even in that case, it’s easy enough to solve: grant permission explicitly under the condition that the assets remain in the context of the game (eg, don’t export them to other games).
Consider other games that explicitly provide a blanket grant for people permission to use their game footage in videos (Team17).
That’s a different legal problem as there are situations where if you don’t protect your trademark you can lose it. But I’m not a lawyer and don’t know if that situation would apply to mods.
Even in that case, it’s easy enough to solve: grant permission explicitly under the condition that the assets remain in the context of the game (eg, don’t export them to other games).
Consider other games that explicitly provide a blanket grant for people permission to use their game footage in videos (Team17).