I have a few ideas but that’s all they are. In an open brainstorm, it is very likely one gets the really bad, some so-so, and maybe a glimmer of decent.
Every legendary gem should have a primal path. The direct power unlocked should be scaled with the existing benefits and the level at which it is required to unlock so a Bane of the Stricken/Trapped/Powerful, for example, is not absolutely the best.
For Boon of the Hoarder, for example, at its level 50 and when primal unlocked, the new ability reduces the cost of Cube recipes by 2 or 3 materials (as opposed to granting more each time one is found) of all types.
Iceblink, Mirinae, and Wreath of Lightning could give a matching elemental damage modifier like 15 + (.2 * level) [+25% at level 50] and so on.
I’m not saying these are necessarily good ideas.