I like (again) Checkvist's alphabetical shortcuts, like...
l c link to clipboard
a a actions menu
d d set due date
c t clear tags
m m move to another list (or for DL another doc)
My hands are already in place and know where all those keys are and can reach them automatically in sequence as if typing. Modified keys-and especially when any letter key with them is on the left half of the keyboard--like Alt Shft T--are very awkward and I avoid them whenever possible. This is more than just new key combinations I realize, and would likely require far more rewriting that you're prepared to do. Just day-dreaming.
With regard to specific potential keystrokes to implement actions not currently possible, I've already mentioned the "switch to items in the left pane (which is possible now with switch to bookmarks and files and soon, tags-- together with a move up/down to select and go-to in that pane. THen of course switch back to the outline pane or automatically do so when a doc is clicked on.