Well i got an idea about create more interesting challenge using bonus district. You can add bonus district for one month for completing all 10 missions in it. Every mission is interesting one and can have different abilities yuhh missions have abilities like in 1st mission have ability of heal block means players heros are heal blocked for entire time.
(I think you guys noticed same thing in mins simulater after completing 50 wave enemies get smaller as more waves complete.)
Also you can specialy set up heros for missions to get it more harder.
Also there is some abilities i have known
3)get damage over time
4)enemy heros hide themselves
5)enemy heros get shild periodically
6)3 star cumpulsary.
8)critical damage insanely increase
9) periodically revive enemy heros
12)only 1/2/3/4 heros team
13)periodically stun players heros
14)rooting all heros
15)frontline/midline/backline heros only
16) shild heros only
17)support heros only
Well you can add many more abilities to missions.
Players get there district as per there bracket like beginner to legendary .
What do you think about this idea?