Being able to go into your stats and see how many times you have received different types of battle hero rewards.
In addition, when you hit certain amounts of them you receive some kind of cosmetic award (at the very least) to recognize that. Personally I would like to see name decorators that reflect the design of the battle hero reward, perhaps unlocked at 1,000 of that battle hero type.
Other rewards could be given at milestones of 50, 100, 250, 500, etc. Even a recurring reward for every additional 50-100 or so.