Instead of collecting hundreds of thousands of DE in your storage at one time to upgrade a hero, if a new hero is added, why not make it slightly different?
For this hero, you need to stay active and feed it with DE everyday, the amount will vary depends on the level, an approximate range would be 50 DE per day all the way up to 10000 DE per day. The upgrade is instantaneous, which means it is basically a wall.
The hero will have multiple stages (levels), each level changes its appearances slightly (maybe a little larger), and in order to reach that level, you must feed it with DE, non-stop, for a period of time. Levels can take up to 8 days of feeding to reach.
If you decide to stop feeding it for more than 2 days, it's progress will drop everyday until it reaches day 0 of the current level, and you must feed it again to regain that progress. Progress cannot drop below current level. (so Level 4 stays at level 4 forever)
After each feeding is a cool down time, user can use gem to speed up the cool down time and feed it again.
If the hero reaches max level, progress will no longer drop. However, feeding additional DE will slightly improve its stats for a short amount of time.
To help you understand:
Level 1: 50 DE per day, 2 days of feeding to level 2
Level 2: 100 DE per day, 2 days of feeding to level 3
Level 3: 150 DE per day, 2 days of feeding to Level 4
Level 4: 200 DE per day, 2 days of feeding to Level 5
Level 5: 200 DE per day, 3 days of feeding to Level 6
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.
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Level 39: 10000 DE per day, 8 days of feeding to Level 40
Level 40: 1000 DE per day to have a slightly improved stats on Hero.
- Say you are on Day 4 of Level 38, and you stop feeding for a week, then if you start again, you must feed 8 more days of Level 38.





