This suggestion is to make an epic only rune or relic or group of runes/relics that would have benefits that make an epic monster roughly comparable to a legendary in effectiveness.
An example would be a relic that increases monster base stats by 10%, and adds a 25% chance that that monster attacks will add a random torture effect to attacked enemies. Specific effects and benefits could be tailored based on game balancing, and the number of relics allowed per account could also be limited to reduce concerns about game imbalance.
The benefits would be increased viability of epics and more opportunities to develop strategies and synergies for specific monsters.