Last year's approach also seemed to work, more or less, which was to collect individual items for discussion, collect votes unconference-style on attendance/interest (as you propose above), and *then* trying to collect the highly-voted ideas into "topics". We could attempt a +2/+1/0/-1/-2 vote using last year's fine-grained proposals as a starting point, for instance, which could indicate whether devs think "we addressed this adequately last year", "we've talked enough about this" or "there is still work to do in this area".
This year we seem to be trying a more top-down approach which makes me nervous because it seems like it's basically only Rob and I who have proposed topics. I think we're both smart fellows :) but I don't want this summit to be dragged exclusively in the directions of our own obsessions, with highest ranking for those obsessions we share...