Our draft spec for the new Gradebook in 1.9 has been evolving based on all the feedback I've been getting and I'm now seeking wider community input before Moodle HQ start implementing it for 1.9 over the next couple of months.
http://docs.moodle.org/en/Development:Grades
The most important things to look at are the underlying structures for storing data and the mechanisms for transferring grades. Note the new events API which will take care of modules notifying each other of important grading events. Note also that numerical grades, scale-based grades AND outcomes-based rubrics are all supported in this model.
The actual interface is LESS important at this stage because we will have a new plugin structure for displays/reports (as well as import/export!). Extensions like "gradebook 2 plus" will in future be very easy to write as clean plugins, efficiently drawing from the exact same database of grades and just displaying them differently. I hope we start having a lot of community-contributed plugins to do graphing and all kinds of funky things. Nevertheless, your input on these aspects in the spec is very welcome too, as it's going to be difficult to cram all this functionality into an accessible, simple interface.
I'm looking forward to your comments and questions!