During the past couple of weeks I've been working with my mentors Steven, Theodore and Mark on setting the requirements for the new core theme, and a design direction. We're veering towards a highly customizable theme that has plenty of content blocks, with added styling flexibility like in Garland. The wireframes have been done, and I'll publish a cleaned-up version here soon.
With regards to the layout, the intention is to have as many differently-styled content areas as possible, while maximizing customizability by making them collapsible wherever possible. The user would probably have the ability to switch between fixed-width and fluid layouts, although we're still discussing what the best way to achieve this is (still early days - there's plenty of time to worry about the implementation details; right now we'd rather focus on the overall design). I feel this ability to customize the layout would be very useful for a core theme, in addition to colour and typography changes.
The next update would hopefully accompany a fleshed-out design :-)