The GSOC programme this summer provides an opportunity for students to get experience coding and for the Plone community this could mean Plone 5 gaining a catalogue of good out of the box themes.
Maybe I'm being too ambitious but I would love to see really great themes come out of this so I'm trying to pre-emptively solve a problem that I foresee.
Problem: Getting good design out of persons who are firstly coders
Knowing Python, CSS, HTML and JS does not mean you're a good designer. There's a whole other set of skills related to Typography, Grids, Elements of Design etc. and we're not even talking about responsive design yet. Even if we were to get lucky and have an outstanding designer in one of our GSOC students we would still have to hope that they have the time and discipline to get the code and design right.
Proposed Solution A: Crowd funded html themes
Fund the development of a few themes and commission them via somewhere such as 99designs then the GSOC student or students would be given working html prototypes that they would have the summer to implement as high functioning Plone/Diazo themes.
Proposed Solution B: Plone community members "donate" HTML themes
Members of the community that have strong design skills (and time), build out HTML templates in the month before the start of coding. Maybe you already have a really good HTML layout that you built but never made it to production this might be a good candidate to "donate" to this project.
Include a Creative Brief
Regardless of whether we do the crowd funded themes or not, we should do a style guide and creative brief for each theme. Perhaps take inspiration from a few modern themes.