We've had about a year or so with stormblood, and I think there are some lessons from it that the devs need to learn for future content. They are.
1. Core PvE and PvP content is the most important thing of them all.
Even with the serious balancing Stormblood brought to jobs, simplifying them and making them much more player-friendly, this game lives or dies based on its core content. If it is repetitive or uninspired, or balance issues are not addressed, the content can stagnate quickly.
2. Side content is not enough to keep the playerbase happy.
Ultimate is a failure. Bard perform is a failure. Squadrons are a failure. Fashion Report is a failure. Raid replay is a failure
They are failures because they are not strong enough to keep many players into the game. They interest a fairly small niche if even that. This sort of ties into my next point.
3. You cannot design solely for the JP playerbase or expect all players to act the same way.
Things like Raid replay fail because they don't add functionality to western players experiences, even if they fill the niche for JP audiences who don't have as robust third party support. The same thing with Feast; you cannot coast on being the only real MMO with PvP like you are in Japan.
You also have to realize westerners act differently. We have different problems and may need different measures taken. And we are the majority of this game's market; you would lose 2/3rds or so of revenue if we didn't exist.
4. You cannot repeat the same formula too much.
Stormblood is simply too close to Heavensward to be satisfying. It's the same general content as HW, just with a different wrapper. Progression is the same, leveling is the same, etc. You have to at some point vary the formula because people get tired of the same overall pattern. Some predictability is good, total predictability isn't.
5. You must let players test your content.
Doesn't even have to be western, get some JP players to focus group your content, so you can see how players will use it. Eureka in particular just screams "We expect you to do it this way" and we didn't. And the content suffers so much for it...it's just an afk-fest where you are handed gear.
Those are my thoughts.