One is, creating a common platform for apps to share data, live data, with one another. By live data, I mean editing a spreadsheet in Excel, and seeing it change a chart in Photoshop, instantaneously. Not after saving the file. Not after hitting a refresh button somewhere.
I know, this has been possible for decades, but there are no apps taking advantage of it, mainly because these things need a common platform with well-defined standards.
This is something that OSes are best positioned to implement, but none do.