I think each different character should have different strengths and weaknesses. Like maybe one character has a high health and damage but also runs out of hunger quicker. Or a character that gathers resources/moves faster but deals less damage and has less health. For challenges i was thinking something along the lines of "Shorter days" where the days are obviously shorter Or "Resource scarcity" where key resources needed for survival like grass and trees are extremely scarce. Or you could also have "Twilight" where it is constantly dusk after 5 minutes of playing. By the way i think completing challenges should yield some form of reward like possibly increased exp or knowledge points