I feel a bit sorry for anyone for whom the suggestion of a lich character in a fantasy game immediately brings to mind "overpowered," and not all the cool stories and fun gameplay that could and would result. Even game designers would do well not to be too preoccupied with [PC] balance in single player games, and without intending any offense, I don't see many game designers in this thread. Leave the boring stuff (balance, technical implementation) to the people who're good at it.
More on topic: Yes, I'd love there to be a lich system in place. Can't go wrong with acquired templates, really, especially as many TES protagonists are essentially granted one or more by the game's main quest! Morrowind did this rather well.
Do I think Beth will actually implement a playable lich template? Nope.
As an aside, I'd do things very differently for the playable lich. Immortality (arguably their biggest "feature") could be made a lot more interesting than a simple health regen mechanic, for example. It'd be trivial to create some kind of phylactery system (essentially an in-character, limited quicksave/quickload) that wouldn't help the lich win battles, but would potentially prevent them from dying as easily as a mortal. I do like your disguise idea though, OP.
Integrating your various lich powers (excepting any attribute increases, fixed resistances, and the like) into the leveling system -- specifically, perks -- would probably be the best bet, as that allows diversity within a given template as well.
I like the idea that Liches would have their own perk sets. If Vampires and Werewolves are in, perhaps they will as well.
Anyway, it's a nice idea, I like it. I don't play any evil mage characters though, so I wouldn't really benefit. The idea of transforming into something horrible in a sick exchange for power should serve as the pinnacle of power for evil characters. Although, this bring me to wondering why there aren't any "good" or "neutral" transformations for characters that like to play morally good/neutral characters, but that's another topic for another time.
Thinking about it, if Liches were in, evil characters would have the option to choose transformations based on their skill set. If you are a Combat character, you would be a Werewolf, and if you are a Stealth character, you'd be a Vampire. If you a Magic character, you'd want to be a Lich. I think this idea has some merit, but seeing as this hasn't been done before, I'd say that it's doubtful this would make it in. Heck, the only reason I believe Vampires and Werewolves will make it in is because they have been done before.