An important thing to realise about Open Source development is that nothing is ever finished and nothing is ever perfect. (If it was, we would all be out of a job ;-)). Open source is never a project , but an eternal quest, and the relevant questions for planning are only ever: "What should we do next?" "What are the biggest problems now?" with the focus on next and now. The quest can only proceed one step at a time, and you know there will be more steps in the future.
Of course, you must have a bold vision of where you are all heading, so everyone is moving in roughly the same direction, but that should not involve details. The details of what you will work on next year are better left until next year. You will be a year older and wiser then, and the state of the art will have moved on.
An important aspect of "what should we do next?" is that it must be somewhere you can get to in one step from where you are now. (Some of the steps in Moodle 2.0 have been quite large.) Open Source proceeds best by accumulating incremental change. That is my belief anyway.
1. A project is a temporary endeavor, having a defined beginning and end ...
(This evolved out of a message I was writing to my Google Summer of Code mentee Olli Savolainen. Thanks Olli.)