I came up with an idea about new challenges which unlock new upgrades for you:
1) 1 energy challenge: You have to complete 20 Endless mode levels with ONE energy to get "Energy recharge 3"
2)Time freeze challenge: You have to complete 14 Endless mode levels only with a bow and "FROZEN TIME" to unlock this upgrade in "Endless Mode". "Kick" will be locked.
3)Endless bow challenge INSANITY: You have to complete 10 hard levels with new enemies only with a bow to unlock "Crossbow".
4)I think that a"Laser" will be a very useful thing for "Endless Mode", so just update "Laser challenge" and complete it to get this weapon.