How upfront about the world, the game, and their shared rules should I be?

If I put all the rules out and am very clear about everything... there will be no non-emergent surprises, and I will probably need to design a deeper ruleset.

If I play my cards close to my chest, the game runs the risk of feeling arbitrary and... improvised? Which I understand probably isn't a problem for some!

And... if I do keep secrets, when I reveal them, will people just share them? Will everyone need to get their own secrets so they don't feel left out & also so people aren't disappointed if they share secrets only to discover they are the same "secrets"?

One of the coolest things of the game-through-physical-mail concept is that it's very conducive to the surprise, secret, and discovery elements. I'd love to see that being embraced! Some sort of mystery, perhaps, even.

You're pioneering a genre here! Well, kind of. So do something cool, interesting and engaging with it! 😁