I feel like these need more improvemtns. As you level up it gets nearly impossible to level up which makes people not wanna level up. What should do is maybe create a whole new 3 missions. New baddies and such.
Level 8, 9, and 10 could be lava themed. One of these baddies could be a medium fast, lava slime that when touched sets you on fire the dmg could be like 2-3. Another baddie idea could be a lava monster like the boss of one of the levels. He could be like 50-60 health. (Idk.) and could give you some pretty good xp. That's so far my ideas for this.
And for the people who reach level (whatever) or something could unlock a special level, a zombie theme. With like some special item at the end like zombie arm or a severed head to wear. but this level has to be hard.