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

[Kaurie]

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.....

Hmm, I don't have many hours in WoW, but I felt that my Holy Priest was very different from my Resto shaman (way more so than WHM vs SCH, where WHM vs SCH was more like Holy Priest vs Disc Priest iirc)

Because of the way they've structured the game, it would require taking some risks to add classes that would truly feel different or offer "game changing" elements.

Given their track record so far, I doubt they will do so. Classes will likely remain a visual wrapper to similar functioning skills with one or two unique uses.

In my opinion, it is a far greater risk to keep things similar. The game will rapidly become more boring if things are not appropriately spiced up. I think it would be better for the health of the game to add a DPS that everyone wants and rolls/levels up etc (NIN for example), than it would be to add a DPS that few people like and is basically a carbon copy of [x] job (MCH for example). Keeping things interesting is what is going to keep people playing long term. Keeping with the status quo is what is going to have people leaving for the next big thing.