well my few ideas Are:
1: when there will be big events (haloween, christmas, eastern, etc) player will get 500 diamonds multiplied by the numeric name of the event in the year.
So for example (not all events, just a few mentioned here.)
1st event in 2016 will be Eastern events - 500 diamonds
2nd event in 2016 will be summer event - 1 000 diamonds (2x 500)
3rd event in 2016 will be halloween event - 1 500 diamonds (3x 500)
etc
2: there should be daily lottery where u can win prices (coins, hammers, diamonds, buildings, space, builders, diamonds) each price can be won only once per month, each price will be doubled on sunday
3: any1 who hit the maximum capacity on coins and hammers has 5% on each to get free diamonds from 1 to 100