The best way is to hire the talent and make them part of the team working on the next iteration or paid dlc.
Offering a purchase model for mods opens a whole new set of issues that need to be regulated, compatibility, system requirements, patches along with main game updates, copyright issues, plagiarism, dependency on other mods and their updates.
The idea that adding a purchase model will only increase the quality of mods is pretty naive. See the app store, or xblig. There are some gems, yet is the ratio any better than with the best unpaid mods?







