I've asked players what makes them quit (viewtopic.php?f=42&t=75458), in order to figure out what to suggest to improve the longevity of the game.
From what I've read so far, other than IRL reasons, there's a few key issues that need to be addressed. The common theme seems to be "it gets boring once you've got it all".
Here's a few ideas off the top of my head. Feel free to suggest better stuff!
1. More in-game events
Meteor is the only "event" there currently is.
More stuff like that might involve different fields.
"Whaling season" could be a group of whales spawning all in the same spot, requiring several knarrs to hunt them down safely.
"Dawn of the dead" could be group of Nidbanes spawning from long-dead animal corpses, attacking anyone holding a weapon (or something...)
"Sylvan rebellion" could be Dryads and Ents challenging several provinces around the world, turning planted crops into their wild versions if the challenge is not stopped (or something else)
These are just random ideas, to get the point though.
1.1 Dev-events
If the above examples are too much work, why not host some random dev-made event, like many other games do?
A treasure hunt... A labyrinth in the caves... a horse or boat race... an arena where players get matched up in random pairs...
Prize could be a hat, or some other cool item.
2. Link bigger stuff to seasons to break monotony
Since we have them, why not fully embrace them?
Crops seasonality comes to mind. Having set times to plant and harvest them would make a farmer's life a bit less monotonous, having to prepare for it.
2.2 Hunting "seasons"
Similarly, some animals might be a lot more common in a specific season, making hunters life a bit different as well.
It doesn't need to be linked to the actual seasons though; having times where large herds/packs of animals spawn might be interesting. Maybe groups large enough that it requires several people to work together to get them (or one guy to find a way to cheese them and ruin it for everyone ), all with random quality like in mini-dungeons.
3. Some decay
From spoiling food to rusting metal, through most animal and vegetable things... You wouldn't get to the point where you have enough of something that you could live the rest of the world without getting more of it.
It would need to be timed properly, making it slow enough that you can set aside stuff for later, but not too slow so that you'd notice things disappear over a week or more. Different things would have different timers, and Alchemy could come into play to slow down or remove decay from certain items...
4. Credo rework
I realise this is a huge one, but I feel like Credos could be the key to make the game more engaging. Not sure yet how, maybe making them more dynamic, rather than "do this - get this - forget about it for the rest of the game".
5. End game objective
A bit of a weird idea, but what if the world ended when a certain threshold is reached?
It could either be something players can knowingly achieve (like a sort of project to work towards), or a "secret" counter (like it ends after 1 million trees are cut).
All of these ideas are assuming the world keeps lasting for about a year (except the 5th one, of course)
Alternatively, the world could just be much shorter, pushing players to achieve as much as possible in the limited time they have before starting over.
A 3-4 months lasting world would make it less of a big deal to miss world reset, and allow some people to test out things to improve on the next restart (which right now is at an unknown date far in the future, so most people forget by the time it happens). But that's for another thread.
Keeping up with everything also seems one of the issues, most likely also what's stopping people from coming back after a break. Farming works pretty decently with WWWs, but other industries have no way to catching up.
Again this is less about the individual ideas per-se, and more about finding options to solve the issues mentioned in the above-linked thread to make people want to stick around throught the entirety of the world.