Every world dies after a couple of months, and then everybody wants a reset. How do we solve this problem?
Skill caps
I think the problem is that players invest way too much time grinding up their character stats to compete with everyone else in a race for superiority. When they die, they cannot possibly catch up/compete with the rest of the players anymore and are at the mercy of constant raiding, pwning, and rapeage.
Slowly and surely, this makes people quit one by one, and wait for the next world. Thus, the world dies, the server resets, everyone who survived loses their progress, and the cycle repeats it's self.
I think we can solve this problem by limiting the progression of characters by introducing skill caps. That way we put a cap on the amount of time it takes reach become 1337, and allows for players to be able to start again and reach a point where they can compete with the rest of the players again, discouraging people from simply quitting and waiting for the next world.
This also encourages combat since people would be less concerned with losing their character, making the game less boring. It would prevent people from locking them selves up behind walls all the time for the same reason.
Renewable resources
Another problem is that nodes are static and depletable. It only takes a few weeks for players to discover and use up all of the good nodes, eliminating the discovery/explorability aspect of the game, further encouraging people to just spend all their time hiding behind their walls until they get bored and quit.
I suggest that clay/water nodes change occasionally, so that people have a reason to explore. Also, occasional earthquakes that level out all of the tunnels in an area, and reset/replenish all of the ore nodes in the affected area.
The earthquakes would also prevent people from using tunnels as massive vaults, as their stuff would disappear.
Also, tree saplings should naturally spawn in empty areas, in the same way grass spawns in areas where all the trees were cut down.
That way players wouldn't have to constantly grind out new trees for fuel and boards.