How often is it that you want to perform this move (edit the character immediately above the cursor, but not in block mode)? If you're moving horizontally by a

How often is it that you want to perform this move (edit the character immediately above the cursor, but not in block mode)? If you're moving horizontally by a single space, you have backspace and delete for single-character moves (which are an edge case). If you're intending to add new content in a new line, why not <Esc>O?