While I really like the intuitive Markdown format for authoring proposals and including figures, I found that I would have really liked a PDF upload option for creating new ideas and proposals. Before I was referred to this site by a colleague, I had already used LaTeX to create a PDF of my proposal and sent it out for a first round of institutional review. I found that having to rewrite everything in Markdown was off-putting because I already had a properly formatted PDF that was ready to go. Having things in a PDF was also important for me personally since I had a strict page limitation.
Therefore, I propose adding a PDF upload or document upload button for the idea/proposal creation process. I do not yet know how the annotation phase would work with PDFs, since that might be more complicated, but I think including the upload function as an option would lower the barrier of entry for new users of the site.
For the time being we've made it so users can simply email us firstname.lastname@example.org and we will convert their proposal from whatever format they have it in. A docx or LaTeX file is preferred over PDF, however.