lich, ascension, atronach...whatever it is doesnt matter, it should just be a caster type.
i for myself would love to see some kind of an adaptive tree.
Based on the staff you hold it will change you your skills (Frost, Fire, Lightning, Heal) so that every caster-class can profit from it, just like the Nightblade profits from Vampire passives.

A necromancy tree would also be a welcome addition IMO

There may not have been Liches in Skyrim (although this is likely in name alone considering that the Dragon Priests are about as damned close to a lich as I think you can get without using the name), but Oblivion sure did have liches and if you got the Siege at Firemoth official mod for Morrowind there was a lich to fight there as well. So the lore most certainly supports it (as does official content). Also according to a few websites (I've yet to confirm in game) there are Liches in ESO (NPC of course).

I think having a skill line for Necromancy would be awesome and quite fitting considering the prolific Deadra worshiping going on everywhere. I mean if you can become a Vampire (who were created by Molag Bal in the first place) why not allow us to become Deadric worshipers and study the Necromantic arts?

With that being said I think Lich-dom would likely be outside the normal realms of what you can do in an Elder Scrolls game considering at that point you truly are an undead being that cannot blend in normal society. Necromancy on the other hand would work quite well. Maybe you could become a half-lich or something so that the fighter's guild abilities would work well against you (like with Vamps and Werewolves) for balance? Also becoming a Lich requires a soul...which the Vestige(s) lack

I would also Love to go the Way of the Lich with my Sorcerer.. and btw in Morrowind you can get a Lich without Mod if i remember right..

I have a Dunmer sorcerer character that would be perfect for Lichdom, think this is a great idea for future content. Maybe some sort of questline you can only complete after whatever happens in the Main Quest? The Vestige could possibly find a way of reclaiming his or her soul, then be free to do whatever they wanted with it...

Although technically, doesn't the vestige already have some kind of easy immortality? Now that I think about it, I'm not sure if putting your soul in a phylactery (rather than leaving it to whatever Molag Bal is doing with it) would actually be safer or make you more powerful

Liches have been part of TES lore since the beginning... Well, not sure about Arena but they sure have a presence in Daggerfall(the game) where they are some of the most dangerous foes you'll encounter.
There is even a book in The Rift that somewhat details how to become one, just before you fight a necromancer that has just transformed into one.