@rahul since the exp has been buffed, even without boosters it's still easy to level up now. And that said, rewarding heroes when you reach a certain level would just make it very easy for players to collect heroes. It would take away playing time. Based on your idea what they can do is to reward a key whenever a player's hero reaches a certain level: example: level 25 - 1 common key, level 50 - 1 uncommon key, level 75 - 1 rare key and level 100 - 1 legendary key.
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