As has been discussed recently on the Orca list, we should bind the unbound structural navigation keys.

The community consensus is:

  Anchor - a and shift a
  Button - b and shift b
  Check box - x and shift x
  Combo box - c and shift c
  Entry - e and shift e
  Separator - s and shift s

That only leaves radio button which should be bound to r and shift r, but that would conflict with an existing keybinding (next and previous live region).