While I have yet to have the time to experience foundry in depth, I have played various foundry missions and they are on par with some of the Crypic's in house mission.
These extraordinary authors goes far and beyond to give us regular players wonderful new story and missions to experience. They should be allow to add in some custom objects and voice over if they want to.
Of course there will be worries about abusing the system and copy right violation, so I suggest a premium reviewing system.
For a small fee, authors can have their map and added content go through a review by Cryptic or PWE (or even some part-timer working for them) to ensure the quality of these foundry mission. To reward the authors for their efforts, they will receive 125% worth of zen from the small fee if they pass the review and therefore publish their mission to a new 'Skilled authors' foundry category. However, if they failed the review, they'll get a detail reason explaining why and 75% worth of zen.
So if the fee is 10 USD to have your mission reviewed, you'll get 1250 zen back if you passed or 750 zen back if you failed. (I am going off of how 1000 zen are sold in retail stores for 10 USD)
They can even make an elite-foundry subscription with its own foundry veteran rewards program that unlock additional foundry assets.