Not sure if someone already posted a similar idea (did a quick search with no results), but I would like to be able to edit and manage my data sources directly on Tableau Server. Whenever I want to make a change to an existing data source (edit the underlying query, add new fields), I have to go to Server first, then download the extract to Desktop (can take quite some time if it's a huge extract), edit the query in Desktop, create the new extract (again can take considerable time) and republish to server. In my opinion it would be much easier and faster if I could go to Tableau server, select the extract that I want to edit, edit the query and save. The next time the extract gets refreshed, it will run the new query.
This will also save considerable time when creating large extracts with many millions of rows. You can then start on Desktop and add a few restrictions to your query, so it will only return a few rows and extract creation will be very fast. Publish and schedule the extract on server, go to server and remove the restrictions. Now the hard work of creating the large extract will be entirely done on server in stead of on Desktop.