Transgender is an adjective coined in 1965, according to Wikipedia. That definition assumes some divergence from our "assigned at birth" gender. After that single aspect of an introduction is established, or owned, we still know very little about how any one individual defines themselves.
Getting to really know someone or establishing relationships is a far more fun dance of discovery. Meet one self identified transgender person and we still have met only one. Our diverse community, whether we recognize being a part or not, demonstrates a single adjective will never be enough.
There has yet to be a definition we all agree on. Perhaps, what our personal definition of transgender means to us is another question your theory asks? Room for everyone is part of my theory and understanding of gender.