I think I'd enjoy something without ARG elements more. Being part of a story or interacting with a world is more compelling to me than straight puzzle-solving. I like puzzles too but I don't feel the need to uncover a hidden world through math and analytics.
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I don't mind getting mail from strangers, but I think a letters page might be fun too - if players wanted to send a message to everyone, they could send it to you to put on a shared page / pages.
Maybe there would need to be a character limit - but it would be fun to look at a page and see someone said "DONT GO SWAMP" and think "huh? There's a swamp?"