My idea is that players would receive consumable stamina items as a potential drop from campaign missions/Time Rift etc. I played a certain Facebook game for a very long time and the game awarded you for continously playing it. And i would like to play Heroes Charge but when i run out stamina i can't.

Maybe create items that restores stamina like this: - Small stamina boost (10 stamina restored) Medium stamina boost (20 stamina restored) and finally Large stamina boost (30 stamina restored).
It's not much but if you keep collecting them then you can play the game more and you don't have to put away your phone because there is nothing to do.