Just create the ethereal item effect - have only effect in Greater Rifts: And create a condition for GR: While using ethereal weapon gems will be upgraded just one time.
Why this ? With that we would have the Ethereal Weapon Memory. So we would remember we played this season. And when we miss it. We can play and fulfill this nostalgy again.
Since we have s25 here on ptr and our seasonal chars from 24 transferred to non-season without ethereals, i suppose, the topic is closed. i dont thinks theres room for negotiation at this point.