I have been giving some thought to the possibilities since the grain of encouragement offered by this BG resurrection has made it reasonable to do so.
I think the ideal model would be licensing a free creation tool that would allow the fans (as well as pro developers) to drive the content creation process. The amount of quality mods and overhauls created in the past for BG and BG2 should demonstrate how effective this model would be, especially when you think of how tediously the mod creators had to hack in their content as they didn't have any slick GUI based modding tools to work with...Imagine what could be done with power tools for the job.
I think that the mods created should go through a vetting process where the wheat is separated from the chaff much in the same way that the app store vets new apps for tablets (ie..they have to work and have a certain amount of quality to them) I imagine there would be a long line of beta testers just chomping at the bit to test these new adventures.
The thing I believe would really make a never-ending supply of new adventures is offering a percentage of sales to the modders who worked hard enough to create a quality mod. This incentive would offer real encouragement for hard work and underscore the need to create quality content...That is, if you make a great, epic adventure, better than all the rest, your fame and fortune will rise accordingly.
The right tools, quality control, financial reward...And me never leaving my computer chair.