Some ideas:
1- Giant meteor.
In missions that that have over 100% difficulty, a giant asteroid will appear once every wave. Like this:
When destroyed, you will obtain 50K points and an atom (firepower). ( ̶i̶f̶ ̶t̶h̶i̶s̶ ̶g̶e̶t̶s̶ ̶a̶d̶d̶e̶d̶ ̶p̶e̶o̶p̶l̶e̶ ̶m̶i̶g̶h̶t̶ ̶s̶u̶g̶g̶e̶s̶t̶ ̶s̶o̶m̶e̶t̶h̶i̶n̶g̶ ̶l̶i̶k̶e̶ ̶a̶ ̶s̶e̶n̶t̶i̶e̶n̶t̶ ̶m̶e̶t̶e̶o̶r̶)
2- Customization in Heroes Academy.
You can customize your ships in Heroes Academy, for a cheaper price.
3- Login rewards.
For every 30 days you log in, the keys you will get will increase, example:
0-30 days: 20 keys.
31-60 days: 40 keys.
61-90 days: 60 keys.
And so on…