I'm with Greg on the idea of an open-ended "Clear your workbench" GB (which is generally considered a Themed Build). However, I wonder if it wouldn't be a good starting GB, maybe a touch shorter than the others, to clear away the unfinished stuff from the previous year.
My problem is GB's that run over the Christmas/New Year period, when many of us have to pack our stuff away for a couple or three weeks before being able to get back into modelling. I'd be happier if the GB season ran from mid-January to mid-December, with a month's hiatus (say between December 15 & January 15?), because getting the mojo back can be a real struggle half-way through a GB build (See previous paragraph).
I, also, think that the Racing GB & Civvie GB could easily be rolled into one GB, with the possibility of separate votes if there are enough entries in both sub-categories.
My 2c.