It's not a "missing" category, so much as a lack of proper descriptors. Just about everyone is "TG" in this forum. lol.
I don't know how true this is, but don't Eskimos have many names for different types of snow? Years ago when there weren't any forums or they were just starting out, most people believed the CDing was just about putting on a dress and if someone wanted what they called a "sex change" operation then they were TS, but more importantly, people had a hard time understanding anyone who wasn't at either end of the trans spectrum. We know a lot more about this now, have a look at the gender page in Wikipedia and also the 156 source material references at the bottom of the page. Unfortunately the words that we use haven't grown and expanded with our knowledge of the gender spectrum.
http://en.wikipedia.org/wiki/Transgender
Honestly, the terms "crossdresser" and "transgender" are catch-all terms for a variety of different expressions, needs, and motives for cross-gender expression. Just saying someone is a CD or a TG is no more descriptive of their gender identity than the terms "francophone" or "anglophone" specify nationality. The term "transsexual" is more specific, it indicates someone who completely and totally self-identifies as the gender opposite than birth, but even then there are layers (and disputes) depending on whether someone wants or can afford HRT or SRS.
Another huge problem: different groups of people have their very own personal opinions of what the very scarce individual words mean based on popular usage, and they either adopt these ambiguous general terms as describing their own particular situations, or they assign those words to people who are not like them. lol. Also, meanings unfortunately become popular through usage without people taking the time to inform themselves of research. :p
In order to get rid of this "black hole" you speak of, people need to identify themselves better than just using one word, until our vocabulary that describes the various major categories of the trans community has expanded and has been universally accepted.
A pretty good start in identifying the major layers of Trans is Harry Benjamin's six-layer gender scale. He devised this in 1966, more is known now and also some of the terms have changed through popular usage, but even then there are overlaps between the layers for some people:
- The Benjamin Gender Scale: http://harrychart.goiar.f-m.fm/Origi...ginalChart.jpg
- The Kinsey Sexual Orientation Scale: http://skew.dailyskew.com/uploaded_i...INS-774322.GIF
(to correlate the third row in the Benjamin Gender scale)