It would be cool if certain essenses were tiered, for example the essence of the predator will always be better than a flat attack speed buff, no matter the circumstance. If we could get a chance after a boss (or alternatively as a new vendor) to combine essences for a price to save slots or have a use for these more basic essences outside of scrapping them it would be awesome so for example:
1 essence of the predator (legendary, 100% quality) + 1 essence of wind (common, 100% quality) = essence of the wind predator (legendary, 75% quality)