Here are a few suggestions:
1. Increase the drop rate for keys, and make it so that we get better keys for harder stages. Enchanted keys from Guardian's Crossing onward, and Dark Prince keys from Zephyr Plateau or so.
2. Make more fuse-able epics. They could be a bit weaker than the ones gained from chance chests but we need epic armors that can be gained through hard work and grinding.
3. Make it so that if someone fuses two higher leveled armors they have a better chance of getting higher starred armors. For example, if you were to fuse two fully leveled legendary armors then you'd have a higher chance of getting an epic armor.
Maybe some of these suggestions have been said be but I'll say them anyways. Hope these will be considered. :D