Originally Posted by
Qualia
That's true, but the point isn't what Azshara could be. It's what she is right now as we know it. Otherwise, we might as well argue that every antagonists (from Hogger onwards) can be powerful enough to be more powerful than Garrosh - just have them stumble upon some convenient massive power-ups (Sword of Sargeras' fragment, mysterious Titans' artifacts, the new Azeroth's wounds, etc.) and there we have it. Blizzard might hype her up and give her some empowerment if they want her to be final boss, but I'd say that for now, her level of power - based on what has been shown to us - doesn't guarantee a slot to be final boss, so it isn't that bad to have her to be the final boss of the penultimate tier of the expansion.
And no, that wasn't what Mannoroth thought. He thought that only his masters could have proven to be superior to her, so she wasn't at their levels yet. This was when she still had full access to the original Well of Eternity (one of the most powerful source of power around, the only one provided enough power to summon Sargeras so far). Now the original Well has already exploded, she lost those empowerment, but gained N'Zoth's instead. We don't know how powerful she is right now - she might be more powerful than she was as a NE, but compare to us, there hasn't been anything indicated that she is more powerful than DW or current Illidan, or on the same level as Archimonde / KJ.
One more thing was that Azshara actually doesn't have much of a story about her. Some people might like her because of her depictions in WC3 manual or WoTA, but in fact, she hasn't done much. Not during WoTA, and not now either. During WoTA, all she did was hanging around her palace or going to the summoning site without really do anything significant (well, she fought Malfurion in WC3 Manual, but that is no longer the case in the triology). Now, I don't quite remember, but isn't she only directly mentioned / shown in the story using her illusions in two or three quests (during Cata and Legion) so far? At least N'Zoth has been hinted to be behind a lot of things Old God / Void related on Azeroth from what recently told in Legion / Chronicle. All in all, she isn't depicted to be extremely powerful compared to us (yet), neither does she has an intriguing / interesting story to carry her. I'd say her position on the second tier isn't that bad.