But again, it's all too likely this system would leave a lot of people unhappy. Free mods devalued, modders in competition rather than cooperation, all the woes of capitalism. I think a system of paid user content could be possible, but the game and whole company really would have to be designed entirely for it.
I must admit I'm not sure it could ever really work: I think it would so fundamentally change the nature of the hobbyist mod-sharing community that we see today.
And that's before you get to the gnarly problem of other people's work being used without permission, which is an unfortunate reality in some of the grubbier corners of the modding world (I've noticed even the very few things I've uploaded, no matter how small, often end up being swiped by someone else and used without attribution of even having their "I made this!" mark stamped on it) and I suspect would rapidly get worse if money is factored into it.
