To answer this question I made a script to find all possible combinations of lines that could form a digit. I used the LED 8 (Like on a VCR) to base all possible numbers off of. There are 128 possible lines, but obviously some work better than others. To that end I decided only symbols where the lines had neighbors should be allowed (thus a line at the top and an o at the bottom are out of the question). I also decided that it should use the full height of the space provided since all other digits do as well. From here I made a little picture of each symbol.
Check them out:
(Note that some don't qualify as possible digits because not all lines connect together, such as 0110110, 1011010, 1101100)
Now then, which two would you consider unique enough to use as two more digits? heh