> allowing all players to bid on every single unowned property that a player lands on (sometimes with some kind of addition such as the landing player getting the right of first refusal at twice the price, or something, but never simply letting them purchase at face-value).
AFAIK this isn't a variant, this is more or less the actual real rule of the game (or something halfway between). If a player refuses to buy something when they land on it, it's supposed to go up for auction with no minimum bid.
No one plays that way, but it's the official rule.