How can they add more classes and make them unique and have their own flavor?

[RiceisNice]

Not sure what you two are talking about, but atm WoW has loads of issues with class identity. They might fit everything in this amazing story and give you amazing abilities, but for example healers, especially now in Legion will be all the same. Same mechanics, same abilities.....

I agree with this. During cata/MoP there was very little difference between the gameplay and aesthetics of a marksman and survival hunter. Both used a ranged weapon, both shot things and things died. Legion is doing an entire overhaul of that though where marksmanshot foregoes the pet aspect (being exclusive to survival and beastmastery now) and survival being a melee trapper. Can't speak on healers and tanks though, but the class idenity has mostly been an issue for classes that had multiple dps roles (arms and fury warriors for example)

Fire monsters weak against water/ice, absorbs fire attacks, ice monsters weak against fire, absorbs ice attacks, hard scaled monsters impervious against slash attacks, flying monsters weak against shoot attacks, etc.

The first time when I played this game, I was surprised that elemental/attack properties was missing, because in my opinion THAT is really what makes every class/job, and frankly, the entire game unique. Without it, there is really no point in which class/job you choose, because every kind of attack will always result in the same outcome.

Then you're going to run into problems where you (don't) want X jobs because of those elemental attacks. And not to sound rude, but I don't think you've played BLM very much if you think that fire, blizzard and thunder spells result in the same outcome, or any other job for that matter because they'd have other abilities that add onto their playstyle (such as BotD, ninjitsu and MNK stacks)