Google Docs' source code displays a message that revealed some important upcoming features: "Coming soon: Third party applications, cloud printers, and sync devices".
Google Cloud printing enable apps on any device to print documents. "This goal is accomplished through cloud print service. Apps no longer rely on local operating system to print. Instead, apps use Cloud Print to submit and manage print jobs." Google says that printers "are treated in much the same way as documents are in Docs". Cloud Print will be implemented in Chrome and Chrome OS, so it'll probably launch in coming weeks.
If you can synchronize your calendar, contacts or mail, whyn't synchronize documents stored in Google Docs? Google already offers some APIs for uploading and downloading files, but they're limited to documents, spreadsheets and presentations if you don't use Google Apps Premier Edition.
Docs has apps for editing documents, spreadsheets, presentations and for viewing PDF files, but what about other file types? There's no photo editor, no video editor, no app for editing PDF files or for extracting files from archives. 3rd party apps could solve some of these issues and make Docs even more useful.