@Obeliske
So I have very minimal time with this game but love theory crafting in any game so I've thought about this too. In this game it feels like if you try to play 100% purely leaning into one class you won't play as effectively as they would in WoW just due to all the changes.
That said it seems like there are enough synergies to make it work like one class but by taking other talents, skills, or even getting specific mystic enchants.
The game feels more like Path of Exile than it does Diablo (what WoW is closer to) where you're really not making a class but more a specialization of the "Hero" class.
For example currently I've got a main focus on Shadow damage with instant cast dots from lock and shadow priest but I use lightning bolt mixed with a legendary mystic that gives me Water Nova (and changes frostbolt to waterbolt, I dont have FB tho) that douses enemies and amplifies my lightning bolt (and any other lightning).
That plus Hemorrhage and Recuperate allows me to be mostly a caster but apply a dot and get some healing and follow with Frost + Water nova as I get distance.
I saw a few enchants that enhanced spells based on if they were ruptured or not (I think). Overall with my limited time it definitely feels like a Melee Caster could be viable, there just might have to be more experimenting with other class mixes before getting it to where you want it.