I was introduced to transformative character classes by 3rd edition D&D. You know what I mean here - a 20th level monk becomes an Outsider... a 10th level Dragon Disciple becomes a Half-Dragon... that sort of thing. Transformative classes might have existed before then. I suppose you could say that in plain-old (Basic-Expert-Etc.) D&D, every class was transformative in the sense that you could become an Immortal. Of course, when everyone is special, no one is.
In any case, I thought these things were weird at first. Then, one day, I read the bit about gods in D&D generally being (as a prerequisite) 20HD Outsiders. It hit me immediately - "Hey. Monks, when they hit 20th level, are 20HD Outsiders." That was a pretty cool realization - that a life dedicated to introspection and perfection of onesself could be a shortcut to divinity. It opened my mind to some of the possibilities of these classes. Around this time, I was introduced to other classes of this sort: the Spirit Shaman (in Complete Divine) becomes fae, the Dread Necromancer becomes a Lich, and the Green Star Adept (in Complete Arcane) becomes (strangely) a construct. There are others as well.
I also decided to play one of these classes. I created a character based around the idea of self-evolution (I named him Darwane) for Jeff's crazy-high-powered-gestalt game. The gestalt thing allowed me a lot of freedom to play with classes. Among other things, Darwane had 3 levels of Human Paragon, 10 levels of Dragon Disciple, and 3 levels of Half-Dragon Paragon (mixed in with a few levels of Monk, Fighter, and Paladin and a bunch of levels of Sorcerer) before he was done. He was a ton of fun to play... and not just because he essentially one-shotted a two-headed, half-dragon (x2) Tarrasque.
I'm feeling like I am rambling today, so you are going to get some half-baked ideas on where transformative character classes could have gone in 3.5 D&D:
- The Druid. Druids are arguably one of the most overpowered classes in the game. Having played one to 20th level (without trying to optimize), I can attest to this. I could see splitting the Druid up into, say, three transformative subclasses aimed, respectively, at transforming into a fae, elemental, or plant creature. Wild shape would be altered for each of these, beginning with short-term, partial transformations.
- Feat trees. There are several feat trees that touch on the transformative such as Fey Heritage, Fiendish Heritage, and Draconic feats. Why not add an option to make these actually transformative? ...or have more of them? I could see a Celestial Dedicant exalted feat tree that turned one into an Outsider, for instance...
- Divorce transformations of this sort from creature type. The creature type system in 3.5 is pretty odd, anyway. When you want to give traits of a creature type, go ahead. Otherwise, just say something along the lines of the Elven Blood property of Half-Elves (for example, For all effects related to race or creature type a 20th level Druid of the Green is considered a Plant).