I think any thoughts about doomsday mechanics can be skipped, as the theory of
Terry Pratchett (Thief of Time) wrote:“Some humans would do anything to see if it was possible to do it. If you put a large switch in some cave somewhere, with a sign on it saying 'End-of-the-World Switch. PLEASE DO NOT TOUCH', the paint wouldn't even have time to dry.”
has been proven with the altar of doom: the bits in the announcement didn't even have the time to get comfortely in their new configuration before the altar had been built and activated.
There are different 'endgames' for different people.
Some like to build stuff and at some point run out of things to erect as all has been done.
Some like to trade but at some point everyone has what they need and trade slows to a trickle.
Some like to kill moving things but run out of challenges (PvE) or managed to remove the available mobs (PvP).
Some like to
whatever but are tired of being treated as targets for the prior group (which hopefully got better with the TPonKO).
One big issue I see is that (compared to other MMO) there are not that many sinks in the game: We have the quality spiral (mainly eating metal) that at some point comes to an end for most (except factions) as they deem their stuff to be high enough in quality so they stop upgrading. Food also gets irrelevant for many as they deem their skills hight enough (or the hazzle to make more to high).
Also we have the problem that the motivation of many lower level players can be destroyed by gaining access to immensely higher quality tools (they buy them, or you simply gift them in case you want them gone) as they'll most likely quit after a week or two when they realize that they won't be able to improve on what they have within a reasonable timespan (or ever) and additionally also
will never have a need to upgrade the tools further - they reached the 'goal' and feel that they're done.
Hence I think one approach to think about is removing the ethernal quality of items (make them decay by use, eat resources to be kept in working condition and lower quality over time) and flattening the curve of quality increase toward zero (in the long run). The reasoning behind this is quite simple: if the goal is an ethernal world then the quality can't raise endlessly...