Chris pointed out in chat that groups already are writing up docs for their proposals. However, the problem in my mind is that first we see a slide deck style presentation, then it is followed by a written doc.
I can't speak for Bob, but as someone who heavily adopted Amazon-style paper writing over presentations for much of my internal team, I far prefer reading thoroughly-written docs to watching slide shows. Right now, the details that we get in a lot of the proposals we vote on are not nearly as thorough as what we see in the presentations, and if we tried to replace the presentations with the proposal docs, we would not have much to go on.
A typical design doc that I would review would have information like:
Project goals (possibly including user scenarios this is aimed at addressing)
There's no one exact way to write one, but if we want to move to a writing-driven review process at least for sandbox projects, we should agree on more questions we want answered as part of the doc and length recommendations (and possibly restrictions). I feel like this doc is going to be a hybrid product/tech design doc, so if there are any product managers who want to chime in with further suggestions I'm all ears.