One thing which obviously cuts down the code space of 7 segments is the fact that there's no levitating glyphs. They all touch the floor somehow. Another thing that cuts down on the code space is the fact that there's no disconnected glyphs. All the segments touch at least one other segment. I suppose you could write 11 on a single 7 seg display, but you wouldn't think of 11 as a single character. The last restriction that cuts things down a tiny bit is that for vertical line characters (i,l,1), it doesn't matter if you use the segments on the left or the right. They are still the same character.

Applying these constraints, There are a handful of could-be letters left over. Some are already in use like the upside down A (used in mathematics), Capital Gamma, entailment symbol (used in logic). Others, like upside down and/or backwards F, upside down 4, and -| aren't used anywhere AFAIK.