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In every sandbox game that features A LOT of building(wurmonline, haven and hearth), you run into the problem of the world just blatently filling up. Unnattended structures just decaying, the "old guard" holding down the best locations, and similar. World resets also have an awesome effect of drastically increasing the number of players and activity of current players(I've seen figures of the active online users doubling for a month before returning to a little better than before).
Another factor that I personally see as a large problem is that since structures are around for so long, the developers fall into the the thought set of forcing a bunch of grind to lay down structures. Which means that they also need to majorly buff the strength of structure when it comes to sieges, and forces players to play much much more defensively due to time investment. All in all I feel it creates an environment where competitive play isn't a factor, rather only "effective" play matters.
I also feel that updates have a much smaller impact in building-sandbox games. Essentially when you release something new players are still going to be stuck with all the old assets, updates can also screw economies in strange ways, and a larger update doesn't cause as large of a boost in activity/players because of the feeling of "the entrenched old guard". I would aim for releasing updates with the world reset.
So the best idea I've come up with is a mechanic that allows an informal RvR system. Pitting the players who want to keep the world alive, and those that want to destroy it. There is no formal system to this; players activate no flags, the developers take no steps to organize the players, and players can change sides as often as their fellows let them.
Essentially in the middle of the world lets have a nice sized dooms-day temple that has 1KM of area around it that players can't build around(Goal is more along the lines of 20-30 mins of travel over rough terrain).
Then scattered around the world are 7 orbs that the temple needs to destroy the world. The orbs themselves being a normal item meaning they can be put in inventory, stashed in locked chests, left in the open, and/or hidden behind player fortifications. The orbs themselves serving no practicable use IE: no useful purpose outside of saving the world, drawing legions of players to your location, and keeping your character from logging out/leaving world even if you're not connected(bad thing usually).
All players have the ability to sense the general location of an orb. The longer an orb has been in one place the more accurate everyone senses are. The timescale I would aim for would be along the lines of 12 hour increments.
When the temple has eaten all 7 orbs all; player stats are reset, a new map is generated, and all items are destroyed. Just for the extra drama would probably make something along the lines of a 7 day dooms day counter as well, just to give players time to congratulate/murder/maim each other and let all the old/future-new players know that new stuff is about to happen(IE: time for social media to catch up).
So above is a system that eventually results in the world being reset, though it might need some tuning. For instance an orb only being able to be moved a certain distance every day. Maybe there being a cooldown for how quickly orbs can be destroyed(measured in days). Though to be honest those are things I'd rather adapt for after they become an issue.
Also curious about other peoples thoughts.
Practice doesn't make perfect, practice makes permanent.
"At one point technology meant making tech that could get to the moon, now it means making tech that could get you a taxi."