The game's basic content formula is fine as a base. The problem is that it's become stale and predictable, and has created a sense of apathy in the long-term players who are finding it harder and harder to care about gearing up because they know it'll be invalidated in 3-6 months anyway.

In my opinion this problem has arisen because Heavensward didn't change the gameplay dynamic at all; it was a good expansion in terms of sheer content but there was nothing really mechanically 'new'. In most other MMOs, an expansion not only adds new content but it also adds new gameplay systems; new progression paths, new game mechanics, new types of side content. Rather than adding more of the same they add something totally new that the game hasn't seen before. They add at least one major new type of content that causes the expansion's gameplay dynamic to feel significantly different to what came prior. Heavensward did not do this. The only new system we got was flying, which was neat for all of five minutes until you realise that it makes no actual difference to the gameplay. They didn't actually use the flying for anything. It's just a new way of getting from A to B.

What the game desperately needs is new gameplay systems. They tried with the Diadem but they made a complete mess of it. The Palace of the Dead looks promising so we'll have to wait and see how that turns out, but it's entirely separate from core game content as far as I can tell, which is ultimately going to harm it in the long run.

The next expansion needs to give us a new core gameplay system. Some kind of new way to develop your character. Something new to do as part of core game experience that we've never done before. Given the likelihood of us invading Ala Mhigo in 4.0 I would say it's a prime opportunity to give us a regular world event akin to Aht Urghan's city invasions. If we re-capture the city as part of the MSQ have us defend it as part of the new endgame. That would be something entirely new that we've not seen in the game so far. It would make the expansion feel considerably different to prior content.

We just need something new as part of the core gameplay, not side content that's completely unrelated to our core character progression. That is what expansions are for but Heavensward failed to deliver.