It won't happen. Between how much their games beg to be modded/fixed, how badly they f*#@ed up with Skyrim, how limited modding is in their games and the general quality of mods that come out for Bethesda games it would be risky and I don't think they'd do it. Nobody is going to buy weapons that float in your hands, or clip when reloading, or poorly/unvoiced quests, or areas that look like they were made in radiant etc.. Games where paid modding works generally have a higher level of quality and very importantly, the ability to "show off" your cosmetics. Nobody buys cosmetics for single player games and that is the only thing you can make up to (or above) the quality standard for the game. Anything that doesn't involve unique art assets can just be copied and half the stuff that does is limited by stuff like animations. I see them losing a massive chunk of the modding community and not being able to recoup from the few professionals that are left trying to sell their mods to Gopher and his followers.