daily challenges are tasks that you complete to get rewards.
if you completed a daily challenge then you need to wait tomorrow and complete another harder than previous one daily challenge.
if you skip a daily challenge then you need to start at day 1 again
let's make the game more fun.
example
the missions are winning ladder battles, completing campaign missions, fusing several items, etc and it gets harder on the next day
day 1-2 - reward item box with 3 items
day 3-5 - reward item box with 4 items
day 5-6 - reward item box with 5 items
day 7 - 1 legendary item
day 8 - silver item box
day 9 - 2 legendary items
day 10 - gold item box (too op? then silver item box)

oh and.... supermechs android is a big data hog for now because of video ads. maybe remove video ads but still keeping picture ads and have timer for them, maybe only 10 seconds.