I also agree that Object Oriented Design would work wonderfully well here. The key here is prototyping its shape and then filling in the details once it has taken this shape.
I like the term 'messages' for methods because it reminds me that the objects must send requests, forms of communication, to the other objects. That's always struck me as a great visual help (I'm a visual learner). With that in mind, we could full-well begin the structuring of the project as it pertains to blocks of functionality: a component to handle visual rendering (whether it include Textile, Markdown, Wiki-whatever, or all three), data persistence layer (ORM, anybody?), etc. I'm sorry if I'm being obnoxiously obvious here, but I like to start things out simple and I think it would be a good step towards a product. For what it's worth, M.T.