A game is fun which you have created with the puzzles and the Community building. One thing that has drawn me into making purchases on other games (as others have mentioned) is not just the hope of a high level character, but the promise of one.
One suggestion is for your packages of gems add a guaranteed 5* hero. I have spent $250 on three packages and won’t spend anymore while only a hope exists. If I was guaranteed at least a 5* then my curiosity would be peaked.
Suggestion two, why randomize the elemental purchases? Add additional boxes and let the user choose if they want random or a specific element all the time. Impulse purchases could generate more income, and user satisfaction. Waiting 2 days for another element, I find something better to spend my money on before then.
Suggestion three, the trading or marketplace. This would raise dean of certain heroes and whereby increase the gems needed. This would result in a $ gain.
Just sharing perspectives that I am sure someone on your team has already considered.