If it were almost completely random (for example A rank spawn window starts after 3 hours and lasts for 10 hours; same with S ranks but 10 times more rare 30 hours after death/100 hours spawn window), there would be a much higher feeling of hunting (or just stumbling into one, maybe) and it would be fun, not necessarily effective, not necessarily a good way to get gear, but the cosmetic would actually be worth something (right now, it's just not, anyone and their chocobos can get the gear as it is, makes it downright bland).

The other options would be making S rank triggers more obvious, but more difficult to trigger. And maybe have a signal where the monster dies until he spawns again, just so that we wouldn't have to rely on hunt tracker websites. (Maybe keep his corpse disappearing into the aether gradually until it completely disappears and spawns 5 minutes later, that'd be cool I think)