It would be convenient to have the three toolbars docked together on one horizontal line to save screen real estate. They could have a visual separation, similar to the way panel groups are stacked in Illustrator. An alternative would be to enhance the Customize Toolbar window, allowing items from the Standard and Document toolbars to be added to the Common toolbar.
We'll you won't get it any time soon, no new features are being introduced into DW in the foreseeable future. It's now only minimally maintained i.e., some bug fixes, OS issue fixes etc.
I think we asked for something similar 5 years ago when the new UI was being developed. At the time, DW's developers said it was not feasible.
To conserve space, I created a minimalist custom Workspace. For my purposes, the Standard Toolbar is useless. It contains nothing that I can't do quicker with keyboard shortcuts or from the main Menu.
I keep unused panels collapsed and have only Document & Common Toolbars active.
Hope that helps.