Better productivity-oriented interface (allow dialogs to be modeless, make everything dockable panes)
As a production tool, drop the ribbon and go to proven high-productivity paradigms with tool palettes, such as exist with Adobe products as Photoshop and InDesign. Being able to arrange direct access to tools on a second monitor provides for far better ease of use and understanding of options.
Several generations ago, Excel allowed one to "grab" an area of the formatting menu and drag it to undock from the ribbon (this was pre-ribbon) for constant access. Given Excel's trend for making things easier for the beginner while burying commands used by the advanced user (requiring more clicks), this would be a nice feature to bring back.
When you drag the Pivot Fields Pane so it's floating, it could automatically collapse into a "bubble", expanding again when you hover over it.
The thing is that the Pane is far too big, leaving a very limited space to work when you have complex pivot tables.
Kenneth Barber commented
Most people, when they make a spreadsheet, start at the top left and work their way rightwards and downwards. This means that you will always have data at the top of the screen but not necessarily at the bottom. If anything, having the formula bar at the bottom, on average, puts the formula bar further away from the cell whose formula it is showing.
I'm not disagreeing with the idea of a movable formula bar, but I don't agree with the reasoning.
Choices to place the formula bar on the bottom of the screen, or at least a pop out window that can be placed anywhere.
It's harder to work with the formula bar at the top of the screen on larger displays and copying and pasting feel more fluid when the bar is at the bottom