There isn't a requirement, although impossible-to-guess rules tend to be not fun and may lead to complaining, and I think it's common wisdom that the person who

There isn't a requirement, although impossible-to-guess rules tend to be not fun and may lead to complaining, and I think it's common wisdom that the person who devises a rule is often surprised at how difficult it is for everyone else to guess, and simple ones are generally encouraged. (I think that, even among a group of strong mathematicians, a rule like "say 'lucky' whenever you play a prime-numbered card" is medium-high difficulty.) Some difficulty comes from having to remember a growing set of new rules; when playing for a long time, I think people tended to say "start afresh with the base rules" after every 4-7 games.