Let me first note that there's skins on top of these original ascensions. If you prefer a certain type of magic that you used before, that's automatically available.
The "ascended" classes are based around what already exists with the 3 specs. Here's what you'll be getting:
Please note that a lot of the names are still a WIP, so a lot of these are just gonna be descriptions of what the class ascension is like
Warrior = Gladiator (based on medieval First Ones' physical barbarianism and bloodsports)
Paladin =
Priest = Astromancer: this one's cool. The original magic of the First Ones is astral magic; the Astromancers are an ancient sect of priests that follow the First Ones.
Hunter = Cool mix between mastery of animals (if you're not BM you can infuse beasts into your weapons) and mechanical abilities that originate from First Ones' technology
Rogue = There once existed sky pirate samurai on Azeroth before the titans came, if you could believe it. Obviously, they're not called this, but that's what they are, essentially.
Monk =
Demon Hunter = Essentially a mastery of your demon form. You'll be using it far more often, and you can even transform into a full-on demon. A ton of options to choose from, and can be changed at any time.
Death Knight = Runes and flames; this ascension is super scary. This thing is a tank (not literally), Oh: domination magic was made by the First Ones, and it has a lot of offensive (i.e. attacking people) properties, too
Druid = Druidism and shapeshifting originates from the magic created by the First Ones. A lot of cool different forms that can be taken on from beasts, creatures and critters that are unique to the First Ones. One of the lesser developed ascensions so far, but the models look cool. Still trying to figure out the mechanics of the magic/spells/abilities they use.
Monk = Some cool chi mechanics; you can become one w/ the August Celestials, and there's some fun drunk stuff too. The First Ones actually created the August Celestials with astral magic (ret-con). Also, it's no secret that the Astromancer priests had some strange brews with even more potent effects.
Paladin = Ancient defenders of the First Ones' kingdom. Not much to describe here yet, but there's a different kind of light magic that's used. These guys were the protectors of the upside-down max level region I mentioned above.
Shaman = Another lesser-developed ascension, if I'm being honest. At the moment, it's a mastery over the elements, and you can do a lot more with them (i.e. remember blood-bending from Avatar: the Last Airbender? that's actually gonna be a thing...)
Warlock = What if I told you that you could summon servants from the depths of the First Ones' chambers?
Mage = The First Ones are the reason the the concept of time even exists; a lot of cool arcane/fire/frost abilities mixed with time magic