Search and import documents from repositories
One feature that would make Zotero the greatest app of it's kind would be the ability to search in repositories (ieee, acm, etc), and automatically import not only meta-data but also the papers themselves. There's an application for MacOS called Papers that does this, and it is the only feature I really miss in Zotero.
Now all it's missing is the distributed search in repositories :) I mean, I would enter a term in zotero, and it would search it on chosen repositories and return the consolidated list of results (or a list of results for each searched repository, i'm not sure which of the behaviours would be the most useful).