You're looking at it the wrong way. A class should not be defined by how much of a stat it has, but how it does it's job. A tank should not be defined by how much defense or offense they have, those are just numbers. Think of it this way, if you took two tanks, gave them both the exact same kit, literally identical skills, with just different names and animations, but made Tank A have more mitigation, and tank B have more potency on the skills we'd all say that's boring as all hell. Lets look at the opposite extreme though, both tanks have exactly the same mitigation, and exact same dps, but they both play radically differently. They have different resource management, totally different combos and playstyle. We'd all be praising the design. Here's the thing, since both have the same mitigation and same dps they fulfill that requirement of all classes being viable on all content. You're also not worried about getting multiple of each one in duty finder.
Now with different class design it could mean that skill ceilings are higher and not everyone is capable of performing on a class. That shouldn't be a problem though as long as there are accessible classes. The good players will enjoy the high skill classes and others need to get get good or play another class.
Even when it comes to things like sleep, or CC in general. They could easily define a 4th role of support and it's classes all have buffs and CC and dubuffs and such. Likewise since it's already required, nothing is really stopping them from giving CC to healers and requiring it. They could even implement other types of CC that fit with other classes, like stunning fear: "Your rage filled shout makes enemies so afraid they cannot move or act, effect ends upon taking damage."
It's all there if they want to use it. However, as long as classes are defined solely by how much defense, or how much offense they have, yes you're right there's not much they can really do.