Would it it be possible for each of the heads be different aka copy each rune.
Visually either have 4 smaller heads like now and 1 big head,
or have 4 smaller heads in a circle and in the middle is the bigger head (last rune).
On what set effect it is located at up to you.
As an added possible bonus if people don’t like different elements, the different element option is only on, if you use the no rune version hydra skill.
A different approach could be that it is for the last rune only, considering at the moment the last rune only will be the most unused rune because of the set as it is now.
Basically the idea is what monk has already with it’s innas set and mystic allies boots. Also considering originally who managed these two classes was one and the same person, no idea about now.
Just my two cents.