I just came to a thought... What if parallel walls have to have a 1-tile space between 'em? (probably won't work 'cuz we can put there a statue anyway).
Also, what about destructible cliffs? And destructible village banners and statues (which could drop the authority down a bit more than they give, thus making vaults suicidal).
And i think that sleeping on the ground is a good idea, though sleeping players should have hitboxes (so there'll be no problem with blocking of character's place) and be movable (!), so they can be stolen while sleeping or moved if try to alt block.
Aaaaand how about "only one Haven process on one computer" feature to at least hurt alt users a bit.