Short version: External Transformation (TF) brought out and self-inflicted by slow mental changes. That's basically my fetish. It just kind of seems to fall outside the range of common tf/tg for some reason. I find the change of a human being,especially their mind, in itself erotic, to the point of fetishization, but like the idea of some instinct or urge, some internal force changing them externally within rather than the other way around.
Examples: These definitely make good on this idea, as a start:
http://hypnochan.org/hyp/res/32719.html
http://hypnochan.org/hyp/res/33947.html
http://www.furaffinity.net/view/3689462/
http://www.furaffinity.net/view/3689297
The last two have some incredibly nice introspection (in the details and the latter in the comments) on the ideas of warping someones sense of self-identity merely by introducing them to unshakable values and urges that must be followed. Just like a bimbo/slut, but rather than it merely being sexual arousal and libido making them further into what they decay into it's whatever subjective value the creator uses.