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
1)shild block
2)heal block
3)get damage over time
4)enemy heros hide themselves
5)enemy heros get shild periodically
6)3 star cumpulsary.
7)armour down
8)critical damage insanely increase
9) periodically revive enemy heros
10)auto playing
11)manual playing
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 .
I actually really like this idea, it allows players to act different and strategize more, just like in danger simulator where such effects are already implemented. Great idea!!
Then besides that, what should be the price like GIR asked. I think certain frags for some heroes, like in district 1 it gives Nightingale frags. District 2 it gives butter frags. And for the very last district is gives frags for a 7 star, or instead a new rewards system would be cool, like a store with special shards and the higher you go the more you get. Though i personally think that frags for a certain hero that should be used as of the storyline like i mentioned as it makes progressing the campaign easier for some. Which can be seen in many other games too!
All rewars are one time rewards but still you can reset missions using gold also this is all 3 star rewards only if you not get 3 star in any mission You will get bucks.