I forget the channel, but someone has been streaming btop and grafana charts for the last couple open sessions. It's cool to see, but I don't think anyone here is going to offer you a mountain of cash based on the metrics lol.
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It's a hard problem to solve. FF14 probably? does the best job of keeping their entire world alive. But that's just due to the nature of how they do jobs on a character, and it encourages queuing up for "old" content. Given x amount of new players and churn, the hard requirement to do all the previous content before the latest, etc. But given this isn't a narrative or smash your dailies and log type of game- they have a harder road. New classes and races have been teased, and twinking alts is always a good time, which will help some. But on the other hand, that's kind of a tomorrow problem too. Best bet given the constraints is probably to not expand the world too wide, but to make more dense or vertical (teh deep?) zones that keep the higher levels circulating amongst the lower. Introducing interdependencies amongst the levels would help too, but i can't immediately brain a way that wouldn't be alien.