First reaction: what?!?!
Second reaction will come when I catch my breath.

I think this will prove to be a very rare wish. It will also take a large amount of development to bring about. Wikidot would have to develop a complete copy of the subscriptions package, such that it can be duplicated many times, and would need more human support for it. I don't think that amount of investment could be paid for by the small number of sites that would take advantage of it.

Payment gateways are censorship. The Internet views censorship as network damage and routes around it. So you need to have a service that no one else has. About the only examples of this are magazines, where your editing and writer-base are worth people spending money on. Just a forum or just a wiki would get under-cut.

But if you want to do this, you can do it now. Have one public Wikidot site for people to subscribe from with a PayPal button. Handle payments either manually or automatically with a mail-server. Then give paid-up customers access to a private Wikidot site. Of course you will need a pro+ subscription to get an unlimited number of private members.

I would orgaize this with a compound of my wikidot site ( the "Viewer" to my saleable "content"..) and the download and payable part.. and that I would do on lulu.com .. where your visitors can download digital content too ( and pay for it).