Hello, and forgive me for my totally impossible idea, but...
...what would happen if you distributed the data that is currently processed by the server to the clients themselves?
I'm not really a programmer and can therefore just guess the amount of work needed for such a modification, but it could allow an infinite amount of new players, since every new client could also take over part of the server's task. Since I do not know the server-side code, I cannot know what exactly makes the server lag, but by creating a peer-to-peer network, it could improved. I'm thinking of something like Seti@Home, maybe Haven@Home?
Of course, that would mean greater vulnerability of the game world itself since hackers would try to manipulate the piece of info that is being processed on their computers, so also some kind of encryption must be implemented. On the other hand, how many people in the Haven&Hearth fanbase are capable of such a feat?
I discussed this with someone in-game and he told me to post it here, so I did. I would love to see Haven become more stable and lag-free, as it is the best game I've played in quite some time.