This a premium service, world class. To keep people's interest it is super important to have regular content updates. Team's goal is to be constantly introducing new content, the frequency needs to be determined. Frequency will be more than what people are used to in regular MMOs. Will be working on content to add even before the game ships.
http://www.gamesas.com/topic/1434330-the-zenimax-video-interviews-of-2012/page__st__30
This was off an interview given, thanks to Stonethrow for interpreting it, so it's pretty credible considering how amazing Stonethrow is.
I never really saw much conversation on this bit of information, and it's been out for a long time. I know things are subject to change, but this pretty much sums up their mindset on payment methods. They're aiming for regular content, with even more frequent updates, more than what people are used to. If "what people are used to" is WoW's standard of pumping out a ton of content, then there's no other payment method possible other than subscription.
Based on this piece of information only, not your own preconceptions, what do you think the payment method realistically will end up being? How much do you think ZOS will be asking for this amazing service? Do you think they'll keep to their word and regularly send out new content? What kind of quality of content are you expecting?