Unpredictable or random rewards are more addictive than a predictable schedule.
"Some events that you need to do repetitive tasks to complete have a fortune cookie as their reward, which gives you a random item (the collection of which depends on the cookie, that much you know from its name and appearance.)"
"you can make your own quests which can give you tickets to summon random trainers."
"Open a "Chest" after a certain objective is reached. Said chests claim to have a percentage to give you a new character to play as. These claims are probably misleading."