Functionality for attaching files to groups.
There are many times when documents apply to the entre group of documents, for instance if a researcher makes a spreadsheet summarizing outcomes and methods from multiple papers, it is convienent to have these with the papers.
This has the risk of becoming a storage place for all (or many) project documents. THAT could be a feature, or a disaster 
For me, it would be an added bonus.
Alternately, the ability to export literature (the attached documents and notes) would mean I could simply export them to my own directory for storing reserach documentation and collaborating with others.