Another feature I suggested to Mark a while back is to incorporate an online office suite into the site.  This would allow users to collaboratively edit files without upload/download and "keeping it all straight" issues.  Before I came to groups.io, I used a site that had this and it was great.  Like having your own private to the group Google Drive.  Like OnlyOffice or some other open source online document editor package.  Lot of work to integrate with security model though I imagine.
Toby