Requirments; for this one it can be made into stages, early game, mid game, end game. Certain hours or completions to do a boss.
Difficulty: depends on the boss, give them certain teams or rotating teams to make it a challenge, but only if the rewards change too, at this moment bosses are just giod for a bit of pocket money but thats it. The thrill of a reward poke is a bit off since the new added spawns.
Cooldown: imo 12 days is a good cooldown. With the increasing amount of bosses it takes a while to do them all so if the cooldown would be decreased it would take up most of the time.