It would be cool if they are powered up like a werewolf or vampire, but not any more. Also there should be a way to rehumanify yourself. Like at Meridia's shrine in homage to Morrowind's Molag Bal quest.
Plus it would complete the power trio
Lycanthropy - Power
Vamperism - Stealth
Lich - Magic
NO NO NO NO NO NO AND NO!
I have explained this already, and what you put up there is rubbish. Sure, liches would predominately be magic users (well, only), but vamps and lycans are not those. If you had even played Bloodmoon, you should know that werewolves were very sneaky AND high damaging fighters, that can tear shields and armor like tissue paper. But, you have to remember, the greatest power a werewolf had was its ability sneak up behind someone, and completely wreck them. Of course, they could also go toe to toe in a fight (should sneaking fail) and is able to use speed and strength to completely mangle someone. However, werewolves cannot cast any spells at all, and they cannot loot stuff from bodies, and MUST kill a humanoid every time they transform (unless done with the Hircine's ring, and the hunger is repressed with the ring in Daggerfall). To sum up, werewolves are extremely quick, have extremely high damage, can shred armor to nothing quickly, and are great sneakers, despite not being able to even ready a spell. Not to mention that werebears make their home in Skyrim, I can expect them to have the damage output of werewolves, but trade speed and stealth for health and defense.
Vampires, on the other hand, run a gambit of power. While all vampires do gain more strength, speed, and better sneaking abilities in general, different clans also gain more bonuses elsewhere. Look at the list in Daggerfall and Morrowind to see what I am talking about. There are clans that are more about martial prowess, others that boast excellent abilities to heal, some become more magically gifted, and others have their stealth abilities even further enhanced. The Cyrodiil clan is special, in that they bargained with Clavacus Vile to be able to hide in plain sight, as long as they feed regularly (other clans are perma-monsters). To sum, vampires run a gambit of power and abilities than just sneaking.