This is kind of a bad idea...

It can be something where you battle and if you defeat a certain number of dragons you get a prize. The dragons you use change every time. It is 1 vs 1. Use it once, and it's rank improves. The change cycle changes through events. Here is an example:

Dragon change cycle:
Epic
Legendary
Very rare (20% Epic)
Rare
Rare (50% common)
Epic
Legendary (or 5% heroic chance)
And an Epic (50%), Legendary (35%), or heroic (15%).

The dragons vary for each player.
If you keep using them again and again, they will get better rank and level. Starts at No rank, level 15. Every battle gets you points and every 20 points you get a prize. Main prize at 300 points.