Persistent Selections, epitomized by the WordStar family of editors allows you to mark the beginning and end of a selection in a non-modal fashion, navigate to the beginning and end of a selection using keyboard shortcuts, and edit text in and out of the selection without ever altering the selection begin and end points.
First, two keyboard shortcuts would set the beginning and end of a selection. These could be activated at any time, in any order, and without any dependency on typing in between. So, one could mark the beginning of a selection, and then begin typing from there, editing other code, and various other actions, before finally deciding where to end the selection, which would be accomplished by hitting the ending selection mark shortcut. Then, the editor would highlight this region between the beginning and end of the selection as the region selected, but one could continue to edit any text without altering the region start and end points. In particular, after a region is selected, new text could be inserted between the region points and the selection will "grow" to encompass the newly inserted text, and inserting or deleting text in or out of the region would also likewise have an analogous effect.
Finally, this region could be moved or copied to somewhere else by the use of two other keyboard shortcuts. In the case of a copy, after copying the region, the newly copied text would form the new, current region, rather than the old region, allowing for continued refinements on the new copy, without needing to reset the region to the newly copied text.
A weaker form of this exists in editors like Emacs and Vi, while a good example of this simple but powerful feature is found in the JOE editor in WordStar mode.
As a final note, these persistent selections should be different than the normal mouse selections, which should continue to operate normally. It might be useful to have a shortcut that enables one to select the persistent selection with the normal mouse selection, enabling copying that region to the normal clipboard if desired.